Somewhat of a matter of approach. Most of them keep iterating on the same techniques in a constant arms race. Just so hard to make much more than an incremental progress and differentiate from everyone else.
I like the approach being used by Atriceps (http://www.atriceps.com)-- they're flipping the problem on it's head with a consent-based approach that looks for valid content and reputation, rather than exclusion.
Disclaimer: My company was in the Fall 2015 cohort of the Mach37 Cyber Accelerator with them.