I think both Marijuana and gay marriage legalization started because of a small group of like-minded people that found it acceptable and were able to spread that ideology enough so that eventually the state could vote on them. For those particular examples, competition doesn't really kick in until it's already been experimented with, but then it really speeds up the spread for sure.
Looking at the timeline of cannabis laws [1], recreational use in particular, Colorado basically tipped the ice berg. My point is that the organic spread of new and/or controversial ideas is localized before competition kicks in.
[1]: https://en.wikipedia.org/wiki/Timeline_of_cannabis_laws_in_t...
For the same reason that Google tells Congress 'You need to let us run free to fight those Chinese giants', the US needs federal level regulation to play on the same level as those companies, if it had the guts.
I'll offer a different take: taxation (and budgets) are just the price we pay to society for a basket of services. There's some optimum price / optimum basket. This competition allows different societies to lower their price (taxes) or lower their basket of services based on the democratic needs of that society. Some societies will value job creation more than short-run costs to the budget, and others will not. It's not surprising at all that the city that didn't have a whole lot of jobs chose the former (via the democratically elected government), and the city that already had a lot of jobs (NYC) chose the latter, also via a similarly selected government.
Everyone wins when competitors try different things and find out what's efficient and what works well and what doesn't. Marijuana legalization and gay marriage started as experiments by states and localities, which could find and set the example to be adopted federally.
Everyone loses when an entrenched monopoly (here the federal government) can forcibly impose one way of doing things with no room for deviation.