They don't think they can and their right. The only winners for the Wii are Nintendo in terms of games that sell.
I can't say I agree with this comment. EA has gone on record as saying they're quite pleased with their sales on the Wii, and they've actually allocated more support for the platform. Capcom has done quite well for itself, with all of its titles so far meeting (and often exceeding) their expectations. Sega is basically being saved by the Wii, etc. etc.
No, I think a lot of the other posters are correct. If Epic was willing to put forth a good engine for the Wii, they'd find plenty of developers who are interested. Shoot, Ubisoft tried to modify one of its engines for Red Steel, and another company is trying to "port" Unreal 3 over. They're doing this despite Epic's complete indifference to the platform. If they tried to feed the demand many developers have for an engine of Epic's caliber on the wii, I'd imagine they'd be raking in the dough. As it is, they're just leaving the door open for someone else to claim the platform with what will likely be the highest user base. Their call, I guess.