Why would canon release a big mpix sensor when they are market leaders? Their sales are good and they have the technology for a while. R&D costs a lot, so proper product releases are key to success...
I've wondered the same thing. Obviously Canon knew exactly what the market was for the 5DIII (Wedding and event photographers) and knew they could charge an initial premium because the high ISO performance offered ipeople a tool they could use to gain a competitive edge.
I've never figured out what market Nikon was aiming for with the D800. They had an embedded base of users who were already invested in Nikon equipment, but the market for the D800 was ill-defined at best. Perhaps they found they were losing market share to Canon and assumed it was because of their smaller megapixel count. Not sure it's really worked out all that well for Nikon.
I'm not sure why Canon would feel compelled to follow Nikon off the high-megapixel cliff. I've always felt the only way it makes some sense would be if they just switched out the sensor in an existing body (most likely the 5DIII) and slapped an "HD" on the description (5D HD). That would keep production and development cost low (especially if they just upsize the 18mp APS-C sensor with a few tweaks). But, I just don't see launching a new high resolution "flagship" when there doesn't appear to be much of a market demand for it.
If you think high megapixels are the end-all and be-all, ask yourself why the flagship Nikon has 16 mp and the flagship Canon has 18 mp.