From the book Swedish Death Metal:
Since the 1980's, Sweden has been one of the countries in the world with the most metal bands per capita. I don't think metal music is more popular in Sweden than in other countries, rather the phenomenon probably has something to do with our social structure. Since Sweden is a wealthy country (it used to be, anyway), everybody who ever wanted to play in a band is able to do so. Instruments and rehearsal rooms have historically been provided by the municipalities to a large extent, and in certain places you could actually even get paid by certain associations to rehearse and make recordings. Many cities in Sweden still have large buildings with rehearsal rooms that you can rent for practically nothing.
I was just thinking about this the other day!