In my opinion there's enough awareness of cancer. Together with heart-and-vein-diseases, it's the best-known disease around, the most talked about, the one people are most familiar with and together with heart-and-vein-diseases and STD's, the one where there's the most info to be found on as to what might prevent it and what behaviour might heighten the risk.
I agree with that.
I think part of it comes down to people feeling like they have some power over this disease (or diseases, since there are so many different cancers). You can do all 'the right things' and still get cancer or heart disease, so these events let people do something - even if completely fruitless - to try to gain some control.
Myself, it gives me an excuse to avoid something I don't want to do anyway.