Considering that HVAP-s had far more penetration than othee types of rounds like APCBC and APHE, and usually it required only one penetration to knockout an enemy, so why wasn't it used as main anti-tank round?
HVAP/APCR is based on the principle of using a very hard penetrating core in a lightweight sabot. In WW2 this core was typically made of tungsten carbide. The problem with this was that tungsten is a rare and strategically important metal. It has other more important applications, such as industrial tools. Taking as much as one kilogram of tungsten and firing it towards the enemy, never to be seen again, is incredibly wasteful.
The other issue is not all penetrations are equal. Just penetrating the enemy tank's armour doesn't really do anything on it own. What you really care about when you punch through the enemy's armour is the beyond armour effects, i.e. what happens in the tank you just penetrated? What you want to happen is some kind of damage done to the tank's components and crew. This is usually done through hitting them with metal shards that came either from the armour or the penetrator.
In case of AP-HE, the shell passes through the armour and explodes, showering the contents of the tank with splinters. Even AP solid shot is probably going to make a sizeable hole in the armour and shatter, so there are going to be quite a few fragments inside the tank even though it didn't explode. That being said, the British found out that AP-HE was much more effective than AP shot in North Africa in 1942 when American 75 mm guns came into service. Shot had slightly better penetration in theory, as it would deform less when passing through armour, but the shells were much better at actually knocking out the enemy tank. Troops even took captured German 75 mm APCBC-HE shells and transplanted them into American 75 mm gun casings in order to have that extra explosive effect.
In case of HVAP, all you have to work with is that small metal core. Since it's small, the hole it makes in the armour is also small, and even in the best cases there simply isn't as much metal that ends up inside the tank to damage it as with full size shot or AP-HE. The cone formed by these shards is also relatively small, so a penetration might kill one crewman or disable a component like the engine, rather than result in a total write-off of the tank.
It is also not correct to say that only one penetration is enough. It usually took several penetrations with AP-HE to knock out a tank, with some taking a lot more shots than that. Again, it is highly dependent on where you hit the tank and what kind of damage you do.
To summarize, HVAP/APCR is much more expensive than conventional AP and does not do as much damage to the target when it hits.
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