[Angrist]

Hm I guess you're right. Within 1.5 years we'll get 2 Mario platformers and 1 Metroid game. That's pretty big, we don't 'need' Zelda to keep us busy.

But then again, look at the time between Zeldas:
LoZ: 1986
AoL: 1987
LttP: 1991
LA: 1993
OoT: 1998
MM: 2000
TWW: 2002
TP: 2006

2009/2010 would be a great timing for a next main Zelda. Especially when you consider TP was probably held back for the Wii.
I think it takes longer because they're not too sure what direction to go with it. Holding it back artificially, just not to have too many games in a short time? Madness.