Another reason behind the shift of cricketers to the T20 format is that most of the county leagues in the T20 format include Big Bash, IPL, PSL, BPL, Caribbean League, SA_20, and dozens of other leagues, resulting in a constant demand for players in the T20 format. That is why most of the players retire from ODI format and Test format but do not retire easily from T20 format.
Yeah that can be one of the reason but not many T20 players can play test cricket. Test cricket needs a lot of patience, stamina and skills. The same can be talked for 50 over cricket, whereas in T20 it is a matter of few overs. When players retire they continue to play T20 as it doesn't need them to be on ground a full day. They only need to be on the ground for a few hours which their bodies can adapt to easily.