Because the then fleet manager of flybe on the Q400 fleet stated once that he would prefer taking pilots who had completed there Cpl and IR at the same school. Shorty afterwards they had a visit from the big schools who explained the tax advantages of taking cadets and then that was it how ever many schools you went to.
Since then people have taken it as gospel that you have to do it that way. But to be honest I really don't think it makes much difference to small operators. All the large operators are cadetships only now.