CHL teams get money from the NHL on players drafted out of their league...
The more players a league has drafted, the more "development money" that league will get....it really has nothing to do with making it to the NHL....the CHL gets that money if they make it or not.
Plus, consider the European players now coming aboard to the CHL and getting drafted....the CHL gets paid for those players even if they played a half a season or a full season....helps off-set the fees the CHL teams had to pay to buy out the players original European team I suppose.
The under 20 rule also helps out the CHL by making sure they dont lose much of their top players to the AHL or ECHL before the age of 20, which in turn will continue to help them make money off of their better players.