"What are the main reasons to vote yes?"
Well, the vote has passed but many say that a positive vote for yes is that it will reduce sentences on those in jail for marijuana and also reduce future incarceration. Personally, my feeling was that this should be seperate from the Prop and stand on its own.
The other reason is that people want to support a fully recreational legal status. Ultimately, if enough states do this it will most likely mean that the federal government may deregulate, which is positive.
That said, there is so much unwritten yet on how things will go in terms of real regulation. Taxes may be prohibitive, over regulation, 3 tier distribution systems run by big alcohol. Only time will tell!