Main factor is that interest rates are so low (and have been for years now). With very low interest rates, it would be foolish to get an adjustable rate mortgage since it only has room to go up. Much better to lock in the very low rate for life. Thus, approximate nobody takes variable rate loans anymore.

When I first bought my house interest rates were over 7%, so getting discount on the interest in exchange of the risk of an adjustable rate mortgage made some sense.