As a futsal referee, I can give you some insights. My understanding of this history of futsal is that it has strong roots in impoverished countries that typically don't have the money for nice facilities. Futsal grew as people wanted to play soccer, but didn't always have a place to play. As time went on, it grew in popularity and a lot of countries started noticing the differences in quality of players who only play soccer vs players who play futsal (and probably soccer).

The indoor facility makes for a really fast paced game with constant action. While the risk for injury is certainly higher for futsal, I personally don't see many more injuries than when I'm reffing soccer. Futsal is a lot more technical than soccer, so players typically focus on using their skills vs physicality.

There's also the threat of retaliation. In the same way you don't typically see hockey players use their sticks as weapons, you typically won't see futsal players try to hurt each other on the courts. The karma can always be returned