California is a single party state with a supermajority of delegates. This means that Democrats can pass pretty much any law that they want without any real opposition.

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.

Exactly what the poster said. California is a single party state. There is no choice in most elections. Due to the way elections work, you're usually choosing between democrats who believe the same thing. There is no dissent.