1) scattering of effort (each framework gets less total attention by fewer people),
2) duplication of effort (besides what is unique in each, tons of effort is spend in implementing mostly the same things),
3) Many half-finished frameworks
4) Worse documentation and less books/manuals/video tutorials (see 1, 2)
5) Less chances of getting an answer to a framework problem in forums/irc/SO (again: scattering of resources).
6) Less vibrant ecosystems around the framework (if each of 10 frameworks has 10% of the market, it's not as good a business to invest in making plugins as something that has 30-40%). Same for hosting offerings, support, etc.
So you want variety but not too much. 2-3 big players would be mighty fine. 10 players of equal size, not so much.