Wii U install base right now is tiny. Nintendo didn't get nearly the sell-thru numbers that were expected.
Releasing Rayman now means an artificially limited maximum number of sales.
By waiting until the fall, there will hopefully be more Wii U owners (and thus more maximum possible sales).
All versions of the game will be "new" in the fall. Not "late ports" sold at an expected discount.
And with three SKUs, the marketing costs can be spread across platforms for a higher total spend.
Honestly, I doubt we'd be seeing this delay if the Wii U itself had moved more hardware units. Lack of hardware sell-thru means less third party software.
Having an empty release window very near to launch with a game that's been at the forefront of Nintendo's third party marketing in numerous Nintendo Directs, Japanese publishing, demo kiosk units and one of the only downloadable demos on the entire console means the game is more viable to milk the roughly 2-3 million install base who are still fresh off the Wii U and would love new games to play on it. It certainly helps that the game's predecessor has been fantastically received, is a bit of a cult hit and has gone on to receive fantastic word of mouth.
Even Rayman 3D, a game that was a port of an older game, sold substantially well enough the first couple of months into the Nintendo 3DS' lifespan because there was literally nothing else that interesting on it. It even topped a couple of 3DS charts at some point well after launch.
These kind of games sell well during the first months of a console and usually become slated to be good enough long-term sellers, particularly on Nintendo consoles where being exclusive sometimes yields better results just on principle (see; Sonic Colors in contrast to Sonic Generations, the latter of which came out after everyone had been singing praise to the franchise's return to glory but still performed worse on 360/PS3 over Colors being Wii exclusively). Origins didn't even have that much of a development budget behind it for it to be profitable right off the bat, even with sub-par sales numbers. If Legends could meet those similar ends on a Wii U launch, which it most likely would have, it'd have been peaches and gravy for everyone involved.
It's not going to have that launch hype anymore though, that's just the way it is - people's interest dissipates the longer you hold out on it in favor for other, more interesting things. Just look at Duke Nukem Forever. So much for "people who didn't have patience waiting for it didn't want it in the first place".