If you legalize it at the state level, you create a new market. Nobody is shipping weed in--crossing state lines with marijuana is, as it turns out, frowned upon--so you've got a total (pardon the pun) green field opportunity. There's no incumbent businesses to compete with.
If weed was legalized by the federal government tomorrow, most of the remaining cannabis prohibition laws at the state level would fall away in a few months' time. You could then ship weed across state lines, though, which would mean that large California and Colorado growers would begin pumping weed into these new markets far faster than a fresh startup ever could.
If you're someone with money, it makes FAR more sense to lobby at the state level rather than the federal level. And they are, seeing as more than half of the country has some form of legal weed.
On the topic of FOP though, I think they are spared anti-union practices because they do not pose the typical threat to entrenched power structures which conventional unions do. They in fact reinforce it and protect the interests of the ruling class, so of course the government supports them.
The FOP is one of the worst players out there. There is a horrifying statistic that over 40% of police officers are known to be domestic abusers[0] and yet the FOP has fought for such criminals to maintain the right to have weapons and to give them positions of authority and the power to use force on others in society. Truly monstrous.
In New York State it's the alcohol lobby.
Are the people who have been getting rich just not politically savvy enough yet? This seems unlikely because it's not like they are the original black market dealers turned legit. The cannabis market has been mostly captured by "respectable" white people with access to big money and business accumen who were smart enough to get ahead in what will be a very lucrative market.