#3 To continue making SmackDown his yard
When Roman Reigns broke on to SmackDown during the 2019 Superstar Shake-Up, he made a bold statement and called SmackDown his yard.
This statement has stuck around with fans and has been used in many promotional materials and videos released by the company. This move was to promote SmackDown further and make it bigger, taking in account Roman Reigns' own popularity coupled with the power of the brand.
Keeping that in mind, it wouldn't have made much sense to switch Reigns back to an already loaded RAW roster where there are many more faces on the brand. This wouldn't have allowed Reigns to stick by his statement and make SmackDown his brand.
By sticking to the blue brand, we can potentially see Reigns work up the ladder in the coming months after proving himself during initial feuds. The top of the SmackDown roster is still less populated, and this could allow Reigns to make a big name for himself.
Reigns had a long and fruitful tenure on the red brand, and the move to the blue brand could provide him with more different opportunities. As a face of the men's division, he can provide SmackDown a lot more than many other current Superstars can.