We only have two druids because people really wanted Halsin as a companion, which Larian didn't plan on, originally, and Jaheira is a nod to BG1/2.
As for Minthara not being a cleric:
It makes a lot of sense from a lore point of view. Her as a cleric would not have worked - since clerics get their power from their deity, which they are a devout follower of. The Absolute is not an actual god and Lolth disowned her. So where would she get her powers from? Paladin works the better with her storyline
I do agree that the lack of variety with the companions is a shame.
I haven't thought about that but it's really odd that we get 2 druids kinda makes me wonder why they bothered with halsin.
