Ubuntu is by far the most used Linux distro on server, the problem with CentOS / RHEL is they did not support recent kernel for a long time, so you see very few cloud servers with those distro, especially since Docker / Kubernetes took off. Also RHEL is not free so there is no way it's "popular" outside of very enterprise needs ( HPC ect ... ).
At previous work they moved away from CentOS to use Ubuntu because libc was just to old, actually everything was old.