The big things for our customers isn't just support, it is stability. When you build your systems on RHEL 8 (just released), you will get 10+ years of support and patches for that system. Those changes are all guarenteed to be binary compatible, so the risk of patching ever impacting systems is low. If there is a critical security vulnerability (say Heartbleed in openssl), Red Hat will backport the fix to every supported version of RHEL. That means you get the security fix without (1) having to upgrade the entire openssl dependency and risk breaking something else and (2) without pulling in newer changes that might have their own vulnerabilities.
Going along with the stability, vendors can QA their products against RHEL as a known target.
Last reason for now, if you are in an industry with any kind of compliance requirements, like FIPS, Red Hat jumps through the hoops to get the certification. That makes RHEL the only option in many environments.
Debian never upgrades anything (unless security issue). All packages are frozen at the time of the initial major release. Have fun using curl/htop/nginx/libreoffice from 5-10 years ago, plenty of obscure bugs and missing command line flags.
RHEL actually maintain their system packages.
I have exclusively used the Fedora line in enterprise environments (CentOS, OEL, RHEL).
The point about long term support is definitely #1.