The answer is quite simple.
RTAS and TDM.
Both are based on 32 bit architecture and to rewrite both would be a complete waste of time.
Hence... AAX.
AAX is the answer to our prayers. No more drama about which sounds better, whether or not one will open on your system... it becomes all the same. It can also become 64 bit.
PT10 HAD to be 32 bit, so to be able to still run TDM and RTAS since adoption of AAX will take time.
Mark my words...
The sooner we adopt AAX, the sooner we will see a 64 bit Pro Tools, and other features like non-realtime bouncing and freeze tracks.
Trust me.
But... if you want more information, trust this guy too:
Transition to AAX: A Real Programmer's Perspective