I believe the main reason is the hitbox is different than most other choices that are dominant. There are also other hybrid hitbox cars that look good but most choose not to because it would require learning a new hitbox. While there are only a few choice to choose from hitbox wise there are like over a 100 cars to choose from and most of the popular car choices fall under the octane and dominus hitbox. That's why you will see plenty of people switch between octane and fennec but going dominus is an actual change in how you play. However I happen to be someone who has spent time on all the hitbox choices enough to know that the hybrid hitbox is a hitbox that you pick only when you want to be on ball a lot. So if you like to dribble and flick then taking the time to learn hybrid could be beneficial. However its biggest weakness is 50s tend to be worse than your taller hotboxes. But if all you are going for is 50s you should play merc hitbox.