1. As megapixels increase, the dSLRs start to out-resolve the lenses.
2. New technology, including the newer IS modes.
3. Competition. If you sit still, you fall behind.
4. Part of the delay has been caused by the natural disasters in Japan, which is where the bodies and lenses are produced
1. This an oft quoted urban myth! megapixels would need to increase an awful lot more to even start to outresolve lenses, and even then different colours resolve differently , due to the wavelendth of light.
In resolution terms an 8MP camera has 3456 by 2304 pixels the 5D MkII has 5616 x 3744 an increase of nearly 2.5x the number of MP but resolution hasn't even been doubled - just a 60% increase. This becomes even harder to increase as the numbers get bigger to double the resolution of the 5D MkII the sensor would need to be an incredible 84MP !
What will happen as resolution increases on FF cameras is the flaws in lenses such as corner & border resolution will become even more noticable, and it does beg the question as to whether that is a price worth paying.
2. Are you referring to the new H-IS mode? WHat would be the point in putting that into new lenses when it's designed for the lateral movement seen in macro photography? It makes no difference at all when it's in a normal lens, unless it's used in close up.