Two comments, both purely speculative:
1. Demographics.
It’s conceivable that a pilot who sacrificies whatever salary and lifestyle for an earlier 737 promotion is more interested in the “real flying” that the job entails, and is thus more likely to pass from the outset, from an attitude and dedication point of view.
2. Training department.
I don’t know how it stands today, but for many years there was a general feeling that if a candidate fails a LH promotion, it is the candidate’s fault. But if a candidate fails a 737 promotion, it is the trainer’s (or the training department’s) fault.
Therefore the 737 trainers tended to take more personal interest in getting the trainees up to scratch prior to a check ride. The “system” had something to do with this too - on the 737 the line training generally had a dedicated trainer, who took responsibility, but in LH you could have a different trainer for every trip. It wasn’t that the aircraft is easier to fly (it isn’t), or that the required standard is lower (it isn’t), it was more the system and culture.