Like others have said, I think the Falcon suffered from a few things, but I think the biggest factor is it's iconic to the old guard, not the new, and so the AT-TE outperformed it. The Falcon's a great toy... As a kid, I think I'd love it, however I'm thinking as a person who likes the OT too... So yeah, as a kid I would love it, but kids today think differently... AT-TE's are more popular with them because there's a TV show and a movie last year that featured it. It wins...
THe Turbo Tank has that going for it again, and so I think it'll do fairly well just as the AT-TE did. The Falcon... I still have about 8 of them at my local WM, now moved to lawn/garden, for $99.99... Same ones that were there months before Christmas. Great toy, great ship, but it didn't have support and so it didn't sell nearly as well. I agree with Adam that it's maybe not likely we'll see a new large OT vehicle... If the Falcon can't fly off shelves, I think that says a lot about any other potential OT toy.
I haven't seen an AT-TE now in about 2 months. Even at full price they moved well. I only got my 2nd one using a coupon code at Hasbro because, well, I sure wasn't gonna find it at retail for anything less than $100. That's fairly impressive when you think about it, especially considering how many of the things were at all kinds of stores.