Have you seen those leagues though ? They dont get the same sort of coverage or attendances as the Football League - it isnt comparable. Not to mention, you have the history and tradition side to the Football League.
That idea has already been floated and was very quickly shot down, mainly by the Football League clubs themselves, that said - why on earth would they all - with the exception of Wimbledon - approve this ?
According to the Wimbledon Supporters Trust, Football League clubs have voted to allow the Football League to finalise proposals to allow "B" Teams of Premier League clubs to compete in the Johnstones Paint Trophy.
http://thedonstrust.org/2015/02/23/february-dons-trust-board-meeting-summary/