One stop shop.
Pilots with airline experience have demonstrated they are capable of passing a type rating course and operating the aircraft out on the line. 200hr recruits on the other hand are a training risk. They have to pass the hurdles of groundschool, the simulator, base training and then line training, and the only way of differentiating one applicant from another is their CV and performance at assessment.
Many airlines choose to transfer that risk entirely to the trainee, and get them to pay for the type rating themselves. Flybe, to their great credit, are still prepared to fund the type rating, and simply bond people for 3 years. It is only natural that the recruitment department will do everything they can to reduce the risk of failure, and one way of doing this is to have an easily verifiable training record to hand. That is much easier if the applicant either went integrated or completed their modular CPL and IR at one school (note that ATPL theory exams and the PPL aren't part of the equation).
It's a system that seems to work well, and allows modular students as much chance of landing a job as integrated ones. I would imagine that the requirement would only be revisited if there was a serious shortage of suitable applicants, and that's not going to happen any time soon - if ever.