A serious question deserves a serious answer and you've gotten some good ones from the serious divers.
There are probably a few more reasons as well. Here is one:
The black rubber compound of the Jets or Turtles makes them negative and balances out the extra buoyancy of any gas in the drysuit boots when in a horizontal prone position with the knees bent and the feet up. Their extra weight also allows the diver to change trim by moving the fins back further or bringing them closer.
There are probably a few more reasons as well. Here is one:
The black rubber compound of the Jets or Turtles makes them negative and balances out the extra buoyancy of any gas in the drysuit boots when in a horizontal prone position with the knees bent and the feet up. Their extra weight also allows the diver to change trim by moving the fins back further or bringing them closer.