I can speculate on the logic behind prioritising NTR applicants over direct-entry ones. First, training is an integral part of the business. Given that it's self-funded (and definitely not priced at self-worth), it's another source of revenue. Second, it's often easier to tell someone "forget everything about your previous type and learn this one from scratch" and teach them to do everything your way right from day one than it is to try and hammer any habits from their previous carriers out of their head.
If the latter doesn't sound like it makes sense, I'll tell you why, to RYR, it actually does. Back in 2015, I attended a RAeS event in London where there was a presentation about the RYR training system delivered by one of the highest-standing members of their training department. And one of his main arguments in favour of doing all the training in-house was that external instructors who had been previously contracted to teach RYR pilots had not been punctual enough with teaching things as per the RYR SOP. Quite the opposite, some of them had used the opportunity to "show better ways of doing things" to cadets and even downright insist that RYR was wrong in implementing such and such procedures and policies. Therefore, to protect standardisation, it was decided not to employ external instructors anymore.
Now probably the next step in this direction will be to try and minimise the number of new joiners who have had any exposure to 737 training conducted elsewhere.