Both countries have had very active electronic scenes since the 80s and until today: many of the best known DJs are still from Holland. And it seems that there were/are not only mainstream electro musicians, but also important underground scenes with raves etc. that led to some of the styles I mention in the question. I'd suppose that there was much British influence esp. during the 90s but would be interested to learn more about either or both countries' music scenes. A lot has been written on UK and German (techno) electronic music, but at least I haven't seen much on those two so far.
I can speak only about the Netherlands, having experienced some of it myself. I think originally things took off as much here as it did in the UK and Germany. The first illegal raves and it's music in the late 80's blew over mostly from the UK, but beyond that the British influence wasn't that big in the 90's. Gabber dominated the Dutch scene in the 90's, and gabber was very Dutch. Netherlands' liberal stance towards drugs really helped pushed the scene, things like speed/amphetamine and xtc/mdma became illegal relatively late so the production side of that was very well developed early on. So much so that since then we've been one of the if not the major producer in the world for those 2. But even once those drugs were illegal you could have your drugs tested by professionals at raves for free, that was until 1999. Nowadays you can only have your drugs tested at rehab centers, still for free but.
Netherlands is also a small densely populated and well organised country. A big rave or festival organised somewhere in the country is easily accesible for a lot of Dutch people around the country. So the size of these raves grew quickly, and as a result the organisations behind them became more professional. I lived in a smaller town back then, we took the train to most raves, didn't matter if we had to go to Amsterdam or Rotterdam or Den Haag, it was all a relatively short train ride away.
Gabber primarily started in Rotterdam, in Parkzicht around 1990/1991. I think this should be viewed in the same way as how punk music developed, and Oi! and skinheads. Rotterdam is a port city, with a very big working class. And I think the availability and use of speed/amphetamine was a big force behind gabber aswell. People simply wanted the DJ's to play faster and harder. It blew up quickly and in 91 they were already throwing raves with 10.000+ people. Gabber spread quickly nationwide, some gabber music became big hits in the charts. It became fully mainstream, especially once happy hardcore happened. Gabber was the biggest Dutch youth culture during the 90's, many kids dressing in gabber style. The scene was over commercialised, which resulted in it imploding in 97ish. The older youths and adults couldn't identify anymore with gabber due to how commercial it had become, and the music had changed too much.
That left a big hole to fill. Suddenly all these gabber ravers wanted to go to other raves. That's when trance and techno started blowing up in the Netherlands. Around that time is also when Monumental started with their Awakenings raves. And eventually ID&T and other organisations that previously primarily organised gabber raves started also organising other raves. ID&T's first major non gabber raves Trance Energy and Inner City back in 1999 really helped to establish DJ's like Tiesto and Ferry Corsten. Inner City wasn't just the first ID&T rave that didn't feature gabber, it was also the biggest indoor rave in the world at that time with 25k people, and it headlined Tiesto on the main stage. So they didn't mess about. Oldstyle gabber parties also became very popular around that time, ravers that wanted that old gabber sound and atmosphere of the early 90's. Which is how Q-Dance started, and how hardstyle eventually came to be in 2001ish. There were only so many oldstyle records they could play, so they started playing and making new ones with the same sound and called it hardstyle.
If there's anything else you want to know specifically, let me know. It basically comes down to gabber being the driving force and laying the foundation for today's electronic music scene in the Netherlands. Gabber and it's popularity basically normalised raving in the Netherlands and really pushed the level of professionalism of the Dutch dance scene. Since then the Dutch scene has really matured, there's space now for a lot of events that aren't necessarily that commercial. I'm 38 right now, and I'm going to a festival rave thing next week where the average age will be 40ish, it's a rave that celebrates early 90's rave music with DJ's that were spinning at raves and clubs back then in Amsterdam and who were very influential in those early days.
Okay, so I'm not a total expert on the Dutch and Belgian scenes but I know a fair share of electronic music history in general. The first thing to note is that the Netherlands where always much closer to to cutting edge US culture than other continental European countries because it's always been a central hub for transatlantic trade. So it's not a huge surprise that a new style of music coming from the US would first hit the UK and the Netherlands & Belgium. Another city that had a significant 80s EBM & Wave etc. scene was Frankfurt (which was also fairly close to US culture due to US military presence). But the key difference is that much of the original Frankfurt scene refused to adopt electronic dance music. 80s European electronic music was a very different scene, much closer to the culture of Rock & Roll, more straight, macho and tightly regulated with strict dress codes than the much more gay and liberal electronic music which was historically much closer to the legacy of Disco. While Frankfurt would later become a major electronic music hub in the 90s, the Dutch and Belgians were much quicker to leave the 80s behind. Why? I'm not entirely sure, there are probably historical reasons since Dutch culture is famously liberal & progressive but beyond that we can only speculate.
(Although by the way, while some key proto-trance releases were from Belgium, it would hard to argue that Trance as a seperate Genre from Rave, New Beat & Techno wasn't developed in the Frankfurt area with the key Dorian Grey club and labels such as Harthouse and Eye Q setting the blueprint for the classic trance sound along with MFS in Berlin. Another interesting anecdote here to show how weird musical history can be: I once read that there was a group of German neo-nazi's that adopted the MDMA-rush music that is Frankfurt Trance as the first 'purely Aryan music' and the continuation of Wagner).
It's also important to remember that music scenes don't have to be large to make a huge cultural impact. A few dozen producers and DJs and a couple hundred of local regulars at the parties are enough to invent a new musical style, the currently largest genre in the world Hip-Hop, started off with a couple of kids in New York throwing backyard parties. As an example related to electronic music, in the 80s, there were less than 50 people making and playing techno in Detroit (if we count all the important producers we end up with less than 15 people!) and maybe a couple hundred regular partygoers at the music institute. It was a tiny scene. The Belgian rave sound of the 80s as also developed by an incredibly small group of producers centered around the seminal R&S label. Before 1990 you probably had less than 500 people globally making electronic dance music, it's easy to forget that the scene really hinged on just a few people. In regards to the Belgian sound, what's interesting is that by looking at the early R&S catalogue, you can quite literally hear the sound gradually morphing, the early releases were obviously influenced by the new techno/house sound coming from the US but they all had a very earthy and dark in tone with obvious strong influences of industrial, EBM and new beat. As you go into the 90s, the sound becomes much less obviously '80s' and less distinguishable from the US and UK releases.
But why did these styles appear in precisely this region? Of course we can retroactively analyze and come across connections that seem necessary in retrospective but I think this can often forget how contingent cultural & musical history really is. If Juan Atkins had never picked up making music, techno never would have never happened. If Jesse Saunders had not decided to release "On & On", all the other DJs wouldn't have started making records and maybe house would have never became it's own genre instead of just DJs playing edited disco records. If it wasn't for a few key figures in the UK, acid house might have not swept the UK and electronic dance music would have a stayed a local phenomena of the American midwest etc. etc.
Still, one thing that is immediately obvious when looking at the genres that emerged in the Netherlands and Belgium such as Gabber and 'Dutch Trance' (Armin Van Buuren, Ferry Corsten, Tiesto etc.) is that they are all highly energetic, opulent and highly emotional, often verging into cheese. I don't think it's entirely an accident that these are the kind of musical attributes that famously go well together with MDMA use. The influence of MDMA on the early sound of European rave is fairly obvious in retrospective. The Detroit techno scene with their grandiose, gracious and musical 'high tech soul sound' was almost entirely drug free because there very little recreational drugs available, apart from crack cocaine with it's horrific effects on the local community, which made them highly sceptical of all forms of drugs. Cut to a few years later, they get on a plane and land in the UK and they are playing for 10000 kids with googly eyes and strained gums on a field in the middle of nowhere with the other DJs playing a bizarre mix of breakbeats, cheesy pianos and chipmunk vocals... It must have been quite a culture shock. There is one anecdote in Dan Sicko's 'Techno Rebels' where he describes Juan Atkins (the inventor of techno as I've mentioned) playing in Belgium sometime in the early 90s. The clubs had the turntables rigged to make the pitch go to plus 16 instead of the regular plus 8 and the DJs were blasting 150+ hardcore. Atkins was obviously horrified that the music he had created in a strong nod to European 'high culture' had become hardcore drug noise there.
This is really just amusing speculation but according to underground folklore I have heard from seasoned German ravers, the pills in the Netherlands were always much stronger & more pure than anywhere else... A coincidence? Still, if we look at the UK, they handled their MDMA mania in an entirely different way in the form of breakbeat hardcore, ie. much more sample & breaks oriented - a way of working that can probably be traced back to the prominence of Northern Soul as the prototypical UK club scene. Gabber and Dutch Trance then perhaps are merely updates of the opulent sound of the 80s?
I think most factors have been identified: central hub location for trade, population density, infrastructure, liberal attitudes, entrepreneurship, xtc. Also in contrast with the US disco had never really died in Europe. In the Netherlands most larger towns had discotheques and young people went out dancing to the dance music that was current. Before house there was disco, dance-pop and hip hop. So when house really started catching on there were a lot of smaller venues that also started programming house and a lot of people could get intimate with it. This also means there were a lot of places for beginning DJ's to experiment and get experience. When larger raves took off all this got to the next level fast. The big commercial techno rave and festival is probably a Dutch invention.