Because if they show the 360 version and it still looks really good or anywhere close to the X1 version (which it could), then they're potentially hurting new X1 sales in their eyes. Why spend that much money on a console right now when you could play it elsewhere? I think that's probably why they delayed it too similar to how some publishers treat the PC (Ubisoft) except they delay the PC version as an attempt to get a small percentage of people to double dip.
When a game is delayed only by a few weeks, it makes you really wonder if it's due issues or just a business decision considering that's not a lot of time to fix things if it's truly about quality.
Considering how many people have defended the graphics of TF (subjective topic of course), then they shouldn't have a problem with the 360 version if it's truly all about the gameplay. Gameplay is king so if TF on the 360 is the same damn game with just some minor graphical effects missing, then why bother spending a lot of money to get it on X1? Plenty of people will buy an X1 not just for this game though, but the idea that tons of people will spend over $560 for one game is quite optimistic. I'll get an X1 down the line when it's price reflects the value the more and even if I didn't have a PC to play TF on, I wouldn't spend that much money on one game.