I made similar observations and suggest an explanation. Most people here have already decided that Bitcoin is their hobby or investment - wouldn't you be afraid to see your hobby fall apart and much more your investment?

In https://bitcointalk.org/index.php?topic=78374.0 I have been pursuing similar thoughts, though not with the intent of attacking the block chain or modifying the protocol. The discussion there might be of interest for you as well, since I am presenting some possible, though highly unrealistic attacks, which could lead to similar effects than those you are studying here.