You mentioned (asking) why Baptists don't teach about differing levels of kingdoms, it is because Baptists don't see the different Kingdoms AS levels, but rather on an encompassing area. Everyone is part of one of the Kingdoms, but only those who accept Christ are in the other Kingdom.

Some Baptists (there are over 80 different organizations of Baptists btw) believe the Kingdom of Heaven refers mainly to the earthly kingdom, and the Kingdom of God refers to the spiritual kingdom. Neither are mutually exclusive.

The general belief is that all of God's creation is within the realm of the kingdom of God. It is the Kingdom of Heaven that refers to those who have accepted Christ and are assured of heaven when they die. So the kingdom of heaven lives "within" the kingdom of God, so to speak.