Or maybe... just maybe a little bit... Nintendo finally admitted to itself one of the secret truths of the game industry: post-Christmas games sell well. Blizzard has had the most obvious success with WoW (Jan 2005) and Starcraft (Feb 1998), but other companies have also had success after Christmas. There's only a limited amount of money to go around before Christmas, and there are too many game releases. Something usually has to give in a family's budget. Also, marketing messages become drowned out around Christmas time as there are too many other things vying for consumers' attentions. Marketing costs are higher around Christmas, too, and Nintendo already has to push Mario and Sonic, Super Mario Galaxy, Battalion Wars II, Wii Fit, and a bunch of other games. So maybe they're thinking, "Well, we can stretch our marketing dollar a bit more if we polish Brawl for another month or two. That will improve the game, and we'll be able to advertise it better after Christmas."
EDIT: Oh, yeah, I forgot to mention another possible factor: if there are few other games released in March, then more people will want to buy Brawl.