(1) Identity politics:  This should be self-explanatory.  The hispanics advocate for hispanic rights, the gays advocate for gay rights, the feminists advocate for women's rights, etc., too often it seems that the advocacy is not for principles but for groups.  

(2) The lack of networks to promote up and comers:  The right wing has a bunch of cabals to get their people advanced.  These include the Federalists, the think tanks, the academic networks, etc.  The left subscribes to the philosophically more satisfying but less effective notion that favoritism and crass old boy promotion is a bad thing.  Consequently, the left does not have the kind of unifying networks we see on the right.

(3) Class warfare: The Right subscribes to a rising tide of tax cuts floats all boats philosophy that is appealing across all economic divides.  In contrast, the Left has significant elements that want to pit the lower classes against the upper classes, labor against management, etc.

Ultimately, when you look at the Left you see a bunch of "special interests" with causes that are not entirely compatible as compared with the Right.  It is much easier for the Right to march in lock-step because the things which unify the Right are more basic and fewer in number than the interests which motivate the left.