probably the fear of not living up 2 the hype of being the new star fox or fzero

Because they need games to fill the gaps between big releases.

Because in most cases, Kirby is more mass-market than Pikmin. And in all cases, he's more mass-market than Star Fox and F-Zero.

Meh I couldnt answer it, its the least of my favorites anyways. Some really like it and good for them.

Id rather see them focus on another star fox.

My suggestion: cheap. Not as in it looks cheap or feels or it anything, but is literally cheap to make compared to those other IP's you listed.

Pikmin - console IP which they feel needs to evolve as graphics evolve if Pikmin 3 is anything to go by. Miyamoto is holding it pretty close to his chest too which probably stops it from expanding.

Star Fox - Nintendo are convinced this cannot exist as a rail shooter and to expand it to anything beyond that is going to take vastly more resources.

F-Zero - has always been a decent looker and pushed a solid FPS, which again takes technical work which is more time consuming. I suppose they could revive it as a handheld IP but it feels much more like a console IP.

Yoshi, however, I have no idea about.

I think Rol's suggestion is the main reason, though.

because kirby will remind you of that pink rash you had.... that we all had :)

Likely because they're cheap, so it's a consistent moneymaker, if fairly low-level.

Granted, they could make more in other franchises on a dime (F-Zero doesn't really need to have high production values, for instance. In terms of graphical assets, even GX's tracks were pretty repetitive) and sell about the same level or so.

Because if Nintendo does not use the same recycled IP, we'll have a new IP instead like Wii Music. Nintendo fans are damned forever to love the same franchises over and over again.

As someone has already likely said, its because the games are made by HAL who pretty much only makes Kirby (and helps out elsewhere now and again). Kirby is HAL's baby and it brings in the dough so why not make kirby games? You said that they could be doing other things instead of kirby in the OP but they have yet to prove themselves (other than Smash Bros which I would say is more Sakurai than HAL)

Likely been said already, but Kirby games are developed by HAL instead of Nintendo EAD and they likely have a relatively low development cost compared to most of their franchises.