To answer OP:

it's a union thing.

And please let's not start a discussion about unions.

744 BTW is a sUA only thing (for now).

In a few years (someone with better memory can post dates) ALL pilots will be able to fly 744, 772 as well as 788 and 789.

IIRC it has to do with date PMUA will take delivery of first 788, but I could well be wrong on this.