Because neither Labour nor Conservatives now represent the middle classes, and - although the middle classes may not identify with the Occupy protests - they do feel furious, not just with the banks but with our extractive financial system.
Labour and Conservatives - and let's face it - much of the Lib Dems remain trapped in the old paradigm, that somehow wealth must trickle down, when it quite patently trickles up. No political force is prepared to take on the financial system and hammer out ways of making it humane and effective.
But what the middle classes want, they tend to get:
http://www.neweconomics.org/blog/2011/11/02/why-the-protesters-are-going-to-win