Startups practicing law (rather than providing law-related services other than legal practice) are difficult given restrictions on the practice of law, including legal ethics rules (e.g., things like ABA Model Rule 5.4(d) [0].)

Also, while in other industries if you break the rules of the industry the corporation pays a fine (which is part of the risk of a startup) and the principals all move on with their lives and livelihoods.

Lawyers who break the rules of their industry can be disbarred. While this doesn't completely stop all move-fast-and-break-things in the field, it does limit the willingness of people in the field to engage in it.

Another factor may be that purchasers of legal services tend to be looking to mitigate risk by purchasing those services, which may make them reluctant to consider novel and unproven models.

[0] http://www.americanbar.org/groups/professional_responsibilit...