1. The POTF2 versions are old and rickety, and massively undersized. A newly-tooled AT-AT need not be too much bigger, just a sturdier redesign a la the new Wal-Mart exclusive AT-ST.
2. The AT-AT is an iconic vehicle in the SW saga that appeared in two films and at least 1 comic... so it has wide recognition.
3. It's the 30th anniversary of The Empire Strikes Back. What better way to celebrate than to redo what was probably the most memorable vehicle from that film?
4. You've chosen to resculpt and redesign the AT-AT Driver. What better way to drive sales of that figure than to release a new AT-AT for it to man? Pack-in a General Veers, and possibly either a Driver or a Snowtrooper (one of the best-selling OT troopers to date), and you have a winner!
Remake the AT-AT, Hasbro, with the same love and attention to detail you gave to the new Millenium Falcon from last year!