#1. No brand split = no reason to push Strowman
One of the major things about the brand split was the fact that it made the concept of brand exclusivity have meaning and feel special again. The fact that you could only see superstars on one particular brand meant that when there was a crossover of any sorts, it genuinely felt like it was something big and special.
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Moreover, the brand split allowed WWE to compartmentalise superstars and prioritise a set of them. It gave more opportunities for other superstars to shine and all of that came crashing down when the Wild Card rule was introduced.
4 superstars became many and eventually, WWE simply stopped trying with the brand split. It's a real shame because Strowman was one such superstar who benefited greatly from being a RAW-exclusive name. Now that there is no brand split, there doesn't seem to be a reason for WWE to focus on him as much.