Because the Dems have been so much "better" or at least more advanced than Republican for as long as anyone can remember the entire Democratic infrastructure is so much more built up than the Republican. Good and bad at the same time though. There are a lot of really good Democratic firms and freelancers but there are also a few really good Republican firms and a lot of not so good one.

For example the Democratic field has some really great players in it like Bully Pulpit Interactive, Blue State Digital, NGP VAN, Wide Eye Creative, Target Smart etc... The Republican field has less but still some good ones like i360, Targeted Victory, Push Digital, NationBuilder, etc.

tl;dr - D's have more and historically better firms. R's have less total firms but this cycle we could maybe see them on an equal (possibly better? probably not, but possible) level as D's.