I think the RTG retention problem lately is due to their change in payouts. While they may not admit this, the players certainly can see this. Very hard to win there and the bonus rounds are almost non existent. Also, there are very few RTG casinos that have quick payouts and can be trusted if someone actually hits a large jackpot.
The MG casinos overall have a very good reputation and greater variety. I personally don’t play at them anymore, but I can see why players would deposit there before RTG.