So what are the reasons? What am I missing?

A 767 FO takes a pay cut. A 320 FO gets a $1 pay raise. Go to Houston to learn CAL systems from CAL instructors for what amounts to a lateral move. Are the 737s still in round dial config? Are they still doing Jepp revisions? When they combine the fleet after sli will the pilots be in different uniforms? (Except for the captain's five year pin vs. FO's 15 year pin). Since there will surely be more vacancies there after SLI why wouldn't a junior FO wait and see?