[Suhiir]

I've always assumed the basic criteria is the size of a standard infantry squad for a medium tank, just because it makes "sense" from a game mechanic point of view. Thus a US Army tank may wind up with a smaller carry capacity then a USMC one ONLY due to the difference in the size of an infantry squad.
Note - I said ASSUME.

I believe Don once mentioned that tanks with reactive armor, and engineer/mine clearing tanks are automatically given a carry cap of zero.

With the Abrams and Leo's I believe it's a matter of doctrine more then anything. These days there are LOTS of APC's, IFV's, and other transport so tanks don't "need" to act as infantry transport. Also while the M60 series had grab bars for infantry as part of it's design the M1's do not.

So sure, while modern tanks could carry infantry...would they as standard practice?