Because the cadet scheme has some kind of quality vetting. You have to have at least some ability to be accepted. The Cadet scheme includes the AQC (a kind of JOC course). As a result, the Cadets are given priority in terms of any jobs that may come up.
The ICP scheme, the way I see it is for those people who failed to get onto the Cadet scheme but still have the money to pay for their ratings. The ICP finishes at the CPL/IR stage. CTC are under no obligation to accept you onto the AQC. As a result, ICP people will be given job placements after the Cadets.
As for the ATP scheme, is that really still going?!