RHEL (and SUSE, to some degree) is popular with two crowds - Enterprise Software Vendors who need to have a platform that they claim is "Supported", and anyone trying to run that software. Particularly Oracle and the like. These people typically spend money like it's water, so they're great customers and make Redhat lots of money.
Centos used to be wildly popular as the "Free" substitute for RHEL, but, back around 2009 or so, where Ubuntu actually started to get a reputation for (A) reliable releases (B) rock solid apt-gt support, (C) a whiff of an "Enterprise" backing it (D) Canonical Support, and (E) lots of AMIs floating around (I see 22 for RHEL, 1053 for Ubuntu) - it picked up inertia on AWS. Particular for the crowd that is keen on the .deb linux derivatives.
It sort of feeds on itself - Ubuntu is popular as an EC2 platform, means people become familiar with it, means it becomes more popular.