Why do some black holes spin and some don't?

It's quite probable that all black holes spin. The probability of a huge amount of matter collecting with no net angular momentum is vanishingly small.

The first solution of general relativity's equations for a black hole was found by simplifying the problem to assume that there was no rotation. Thus, it is possible to find, in the literature, papers referring to non-rotating black holes. Later, a solution was found that allowed for rotation, and this is the most realistic physical situation.