Steven Universe is hardly more popular than MLP. Have you seen any Steven Universe-centric conventions being held anywhere in the world, especially to the number that MLP has had over the years?
Steven Universe is a great cartoon series, don’t get me wrong, and is often cited as having deeper storylines and more complex characters than MLP, which is true. As such, it tends to get more critical praise than MLP does. It has more that appeals to adults and teens than MLP provides and it isn’t afraid to tackle same-sex relationships, death, and war, among other serious subject matter.