Well the first question to ask is: was mead a staple alcoholic drink in the Medieval era? The answer is, no, not really. Honey was always expensive and in the absence of cane or beet sugar it was an important source of sweetness in baking etc. so using enough honey to make sizable amounts of mead would have been very expensive. This is especially the case because pre-modern beekeeping was less efficient than modern versions. In the Medieval era, beekeepers often used skep beehives which were basically an upside down baskets that provided a home for bees. The problem with this is that it was often hard to get the honey out without destroying or at least damaging the hives, unlike modern beehives with removable frames. This made it impossible to produce enough honey to make mead be a staple alcoholic beverage, even if all honey was used to produce mead.

Mead would often be drunk by elites (who the bulk of our written evidence talks about) or poorer people on special occasions. To use a modern comparison you see a lot of people in movies sipping on single malt Scotch and I don't think I've ever seen anyone in a movie chugging some Natty Light, but which is more of a staple drink?

That said, you're right that mead has declined in popularity relative to the Medieval era. There are several reasons for this:

1. Competition. Mead was mostly favored as a prestige drink in areas where it was too cold to grow wine grapes. With better shipping and more of a market economy it became easier to ship wine to those areas for people who wanted a fancier alternative to beer. The same goes for whiskey. Once distilling technology was introduced and developed, whiskey and other distilled drinks became a bigger source of competition to mead as a more expensive drink that wasn't available in the Medieval era.
2. Scale. In the medieval period a lot of farms and monasteries would have a few beehives. Easy enough to do, there'd be some flowers about to provide food for the bees and setting up a few skep hives wasn't very expensive and the people around the house could keep and eye on them. But as we moved into the modern period fewer and fewer people had the means to keep a hive around the house, so if they wanted honey they'd have to buy it from commercial beekeepers. And this is a real problem if you want mead as a staple drink. It's a lot harder to scale up honey production than barley (for beer/whiskey) or grapes (for wine). You can simply plant more barley and get more barley. You can't just set up thousands of beehives in an area and get tons and tons of honey. Bees need to eat and there's only so many hives you can place in an area before the bees don't have enough to eat. Of course you can feed the bees syrup but if you feed the bees that instead of nectar and then make honey you'll end up with some pretty shit mead. Of course you can farm huge fields of clover or acacia to provide nectar for large numbers of hives but even doing that on an industrial scale isn't going to provide anywhere near as much alcohol with the same amount of land as if you'd farmed barley or grapes instead. Also having a huge amount of beehives in one concentrated space makes disease a big danger and in modern times bee diseases are a much bigger headache than barley diseases.
3. Adulteration. If you get some unfiltered natural unpasteurized honey it'll look pretty damn weird. It'll be cloudy with weird lumps and almost certainly crystallize if you leave it in a jar. This isn't good for putting on your pancakes so the honey you buy in the store has been pasteurized, filtered, and often mixed with stuff like corn syrup to help it flow and look more clear and bright. Sometimes even if the label says "clover honey" or what have you it'll have some honey that was made with bees fed syrup instead of nectar and/or a shit ton of corn syrup in order to save money. All of these things make the honey worse for making mead out of since mead already has a pretty delicate taste to begin with (with a few exceptions like mead made out of VERY strong tasting chestnut honey) and you really don't want to drink corn syrup wine. Of course good meaderies source pure raw honey and make good mead, but having to deal with a completely different supply chain than table honey makes things more difficult and expensive for them. It also makes it much harder for people to make good mead at home which cuts down on enthusiasm for mead.
4. Difficulty. In my experience making good mead is hard. Honey is a lot worse for yeast nutrition than wort (unfermented beer) so you have to worry about yeast nutrition in a way that you don't with beer. Also if you make a weaker mead, the yeast will simply eat all of the sugar in the honey, leaving you with a very dry drink without much honey flavor. If you put in more honey then you run the risk of producing a lot of cheap booze flavor (especially if the yeast goes nuts on the simple sugars in honey and causes fermentation temperature to rise outside of the temperature range that's appropriate for the yeast you're using) and it can be hard to hit the appropriate level of dryness. Also stronger meads can require a lot of aging, which is just another hurdle. After dealing with all of that, mead is often surprisingly bland. Even a lot of well-made mead can taste like watered down white wine with all of the acid removed. In order to make mead less bland a lot of people add in fruit juice or spices, but then you often end up tasting those more than the honey. You CAN make incredibly awesome mead (friend made some 18.5% mead that he'd aged for years that was one of the most amazing things I've ever tasted in my life) but it's just generally harder to make good mead than good wine or beer. If I were to make mead again in the future what I'd do is mix some honey and yeast into some water with a bit of fruit juice to provide some acid to balance out the sweetness, let fermentation start, sample the fermenting mead every so often and then as soon as it hits a nice balance between not being too sweet while still having a lot of honey flavor, I'd throw it in the fridge and let the cold put the yeast to sleep. This is great for making mead for a specific event but is absolute shit for making a commercial product. I could easily make a bunch of half-fermented mead and bring it to a wedding or what have you and serve it but if I bottled it and put it on store shelves then the fermentation would restart and the bottles would explode. It is my hunch (but just a hunch) that historically a lot of mead was drunk half-fermented in this way (much like makgeolli, Korean sour rice beer, was traditionally served) since it makes everything so much easier from a brewer's perspective but if the mead you're drinking is only part-way fermented then it becomes a huge headache to distribute commercially or to store since the yeast want to finish fermenting.