The unions in California are unhappy with Uber/Lyft employees not being union members since their presence is an alternative (read danger) to some of the unions (teamsters/taxis). Unions have a lot more power with Democrats since they are some of their biggest supporters, so the legislature tends to do things that benefits unions.

Since California is effectively a single party state, the legislators that voted for this don't need to worry about things like being reelected because they are unlikely to actually see real opposition outside of a primary challenger.