I assume it's because it's pretty much the pearl of the region (Bavaria) and if you take this away there is not much left. Also the city is full of pretty rich people who tend to this kind of behaviour.
You see the same in people where the location of the city is similar like Warsaw in Poland for example being surrounded by backwater villages.
Because of just this.
They are the elite who can afford living there.
I was on a business trip in Munich once, took the chance to visit a friend who got me a hand full pre-rolled joints for the remaining days I had to spend in the city. One night after work I went out to the park opposite the hotel and started smoking.
I was suddenly surrounded by police. They are really all over the city and I was stupid. They went through my stuff in the hotel, I gave them the remaining joints and they left. A few weeks later I got a letter saying that they won't follow up on it according to §31a Abs.1 BtMG.
That’s a very strong “might“. Possession is legal for up to 15g in Berlin, and virtually no cop will ever bother you for smoking weed.