It's not all about revenue. Ubuntu has generally had the reputation as the most user friendly and easy to use flavour of Linux for years now. Not always justifiably, and they've made some questionable decisions lately, but it's still been the obvious recommendation for new Linux users, and the one people are most likely to have heard of. A pretty large percentage of technically minded desktop PC users (eg, those most likely to install Linux) are going to care somewhat about gaming.

If someone asks the question "what distro should I use for gaming on Linux", the answer is usually "any is fine, but Ubuntu's probably the safest option". With this decision, the answer becomes "literally anything but Ubuntu". Ubuntu became popular because of good sentiment and word of mouth. And the same geeks trying out Ubuntu on their laptops are the ones who end up making the decisions on what distro their company should use on their servers/VMs too.

I don't know how much general popularity really helps them, maybe it really is irrelevant, and if everyone stopped using it on desktop tomorrow they'd still retain their popularity in server/cloud. But I feel like the only reason they became popular on servers is because they're popular on desktop. There aren't many reasons to use Ubuntu over RHEL or some variant on servers, except that more people will go "oh yeah, I've used Ubuntu before, I know how to use apt".