If Raiden was top tier he would instantly have many players. You're reading way too much into this; D'vorah, Kung Lao, Sonya, Kitana, Scorpion, Sub-Zero, Mileena, Takeda, Cassie, Kung Jin, Shinnok, Kano, Johnny Cage, Liu Kang, Ermac, Quan Chi and Raiden have all been very good at some point in this game's life cycle. You can't say that all of those characters fall into this category of 'characters that NRS buffs because they're popular'. Let's not forget how mad good Raiden was at launch, which technically puts him into that category.
Also remember that it's often times the opposite of what you said: Characters aren't always good because they're popular, much more often they're popular because they're good.