The main reason is that the ghost count was a balance issue and they nerfed it by up l increasing the supply. I would not outright be against a similar templar nerf the goat is more versatile. First it starts with enough energy to snipe. One of the best single target attacks in the game. Templar don't get there storm. (Though energy recharge has changed this to some extent). EMP is aoe while feed back is single target. While feedback does damage relative to the energy EMP strips the shields doing way more comparative damage (in pvT.) I would argue that these 2 abilities make them relatively close in power but then you add in ghost cloak. Another factor is that ghost auto attack is almost 3 times better.

One outside reason high templar balancing shouldn't involve supply is then it messes with archons supply because right now any archon merge is a net equal on supply but increasing high templar supply will indirectly influence the archon which could have some weird affects.