You ask, why 1500 years. This translates DIRECTLY into the patience of God, which is RENOWNED.
How do YOU know why God did that the way He did? We only know of his relationship with the Jews. How did he treat the other 99% of humanity who didn't believe in ONE God?
I don't assume to know God's reasons for doing things the way He did. I simply accept His plans for all of us and life the best life I can.
I also TRY not to judge others...but sometimes I WONDER at some drivers who seem BENT on killing themselves and me.