And that's probably what Nintendo was thinking about, from the jump.
NSMB and its "Wii-cessor" were smashing successes because of their simplicity, fun and "easy-to-get-into-ness". A hub world would more than likely do more to confuse some people who just want to "get to the next stage immediately, like they could in NSMB. So, it's likely that the fact that NSMBWii was going to debut first probably lit up a few lightbulbs on how to do smart design that would garner more casual gamers. And for the rest of us, it'll just end up being more streamlined.
Besides, as some one else brought up on GAF, the "hub world" has done a lot to outlive its usefulness, as far as 3D Mario is concerned. It was awesome in 64, and it did a lot to help you learn the controls and orient yourself, playing as Mario in a 3D world. But since then, it's not as "special" any more, and it's only done more to get redundant as time goes on.
So, for the sake of, again, streamlining, I can dig this.
And that brings up another point.
Peach's Castle. Delfino Plaza. Comet Observatory.
...a Mario-head planet? Really? When there's already another one?
In addition to keeping the player from reaching the meat of the game, Mario hub worlds also have to have a level of grandeur to be respectable. They are very fun to mess around in, always have, always will. These places become iconic; it takes a lot of dev time and resources to make one. Especially when you consider it needs to be glitch-free, any interactivity, and of course having to compose music that keeps the player satisfied for an exceeding period of time.
The key reason for it was probably time-waste elimination.
But, there is still the Mario planet. Mario must somehow access the Galaxy Map. I believe there will be a "hub-world", just not a large, or traditional hub-world. You'll probably talk to that one big blue Luma, and he'll show you the Galaxy Map. Most Mario hub-worlds take time to traverse, and have individual portals to each level. This one may be smaller and have only one entry point, so Nintendo didn't call it a "hub world".
As long as I don't have to walk to everything like SMW and NSMB forever and ever, I'm okay...