Any thoughts on a solid hickory platform under my tt

Hickory is of walnut family, I never tried it. If it does something wrong you will hear it. I use 3" maple under my Nottingham, works well. If I wanted to experiment, the first wood I would try would be rosewood, preferably Brazilian rosewood.

I like solid maple.  I do not know why, but it is very popular.

I am not sure either. But I guess of the inexpensive woods it sounds better in most cases. Cherry, as an example, would probably cost more let alone Brazilian rosewood, cocobolo or African blackwood.
Even Michael Green who is an advocate of resonance control tunable speakers and rooms used to make Justarack racks with thick maple shelves. Mapleshade does this too.

All hardwood is nature's ideal dampening material.  It can absorb and dampen vibrations coming up from the floor.  

I would think the reason why maple is the most popular is that it is readily available and the most cost effective.  So its used the most.  And we have no shortage of it here in N. America being a renewable resource.