Maybe it's because there are more suitable alternatives, such as carbon nanomaterials

There are several possible answers to this:

1) Actually there are studies on the mechanical properties of hBN but of course it's not as "overstudied" as graphene; the question is: how much of these do we need? Is what we have already enough?

2) While hBN may hav a set of nice properties it cannot compete with the "full package" that graphene provides with not only its mechanical but also its electronic and relativistic properties.

3) There hasn't been a hBN hype train and I don't see one coming, either, so nobody will start studying it just because everyone is doing so.

I think the electronic properties of hBN have not been investigated much yet. There has some spaces left to investigate further.

Maybe it's because there are more suitable alternatives, such as carbon nanomaterials