Agile developers expect agile everything. Ubuntu happens to just be a happy compromise between agile and waterfall.
If you look at RHEL, it's 5-10 year old packages, kept alive by an enormous engineering team that backports fixes to old, dead software, which creates a huge pile of technical debt for any developer trying to use "modern", highly modular frameworks.
As far as developers go, In the Ruby, Python, and Node ecosystems, anything that's not the latest doesn't exist. They don't use the system package management, they use gem, pip, and npm. They really don't care about the underlying OS, until it gets in the way, and getting in the way is exactly what a decade-old OS does.
Just to throw out an example. Take some modern ruby on rails application, say Discourse. (discourse.org). Go download a tarball from github. Now try to make it work with nothing but software from the official RHEL repository. Let me know how that works out for you. After you tear out all your hair and skin trying to do that, try to get the pieces from 3rd party repos that will make that work. See how much you have to bring in as far as new libraries and new packages just to make it work. It's still a nightmare even with the 3rd party repos, and that RHEL support contract doesn't cover them - every single piece that's likely to break your application, is now outside of your support agreement, so your company is now wasting at least $799/year for support.
As soon as they start trying to develop on RHEL, the dirty hacks start. There are things missing - the versions of software that they need to make their dependancies work don't exist on RHEL. They end up in a kind of dependancy hell fighting with libraries that are a decade too old to compile their dependancies. One thing leads to another. Eventually, you recreate an entire current OS in /usr/local, or install one piece by piece from 3rd party repositories. At that point, it's not RHEL anymore. It might still say it's RHEL, but it's a bastardized system that looks more like an evil child of Gentoo and Fedora. (both of which are fine distributions by the way, just they aren't meant to crossbreed). The only thing you have left of RHEL at that point are the parts your application doesn't care about, which is probably not much.
Or, you can attempt to containerize with kvm, chroots, or lxc, which, while not breaking the underlying system as badly, means the application is really running on something other than RHEL.
If Red Hat wants developers back, they are going to have to be able to deliver a product with an agressive delivery schedule, maybe even a rolling release, and be able to deliver the kind of support to make operations feel good. That's a whole new territory, that nobody has touched yet, but if they are up to the challenge of keeping decade old software on life support, they are probably up to the challenge of an agile OS.