I would think some of it would have to do with how much memory a customizable Legends game would require? Is a hard drive pretty much a requirement to do the game the way VC wants to? I'd guess "Yes" to that.
That means only the Xbox and Xbox 360 would be able to do the game well, though the PS3 is supposed to have an optional HD -though who knows when it might show up and for how much.
VC may need to design a game that can get by with only an 8 MB memory card for the PS2 crowd, which is just too large a group to ignore when it comes to potential game sales. The game might have to be more limited than the Xbox versions, but so it goes.
There will be many more Xbox/PS2 owners then Xbox 360/PS3 owners for the next 2-3 years anyway, so support for the soon-to-be-old consoles only makes good sense.
Since VC seems to be taking a year off, to use the extra development time to create a customizable game, they must be thinking that ONLY a very deep and well presented custom game has any chance of success versus Madden. I'd tend to think a custom game would be a supplemental game to Madden in the eyes of most gamers -except for hardcore VC fans of course who might never buy Madden. Few will spend $50 on Madden and then another $50 for some custom game. Some will spend $20-30 on a custom game out of curiousity so price will be a HUGE factor for a Legends game. Does that make it worthwhile to develop on the Xbox 360 and PS3 though? As game development costs are supposed to be much higher on the new hardware?
VC/Take2 have a lot of factors to consider here. Which systems to support? How many custom features can we include even given extra development time? How much can we charge for a custom game and still sell decent numbers? Is such a game even worth trying?
BTW, if they are shooting for a release in 2006? Why not aim for a late May/early June release? The NFL draft and free agent signings will have football on the minds of many gamers. In theory a customizable game is NOT locked into the NFL season as far as rosters go. People can adjust rosters as they go along anyway -so why would VC/Take2 wait to release a Legends game right next to Madden? Release two months early, being the first football game on the market -but well after most people have played the 2006 Madden to death and are starting to look forward to a new football game. That can only help sales, and EA has no way to match such an early release without stepping all over the college game and having WAY outdated rosters by the time the real NFL season starts.
Having written all this -I'd still love to see a Legends game for the Xbox 360 at or very near launch.