I think it was more that Ubuntu was easier to use on the desktop, so developers would use that locally or in VMs and RedHat/CentOS on the server. But inevitably, they would also then push to deploy on Ubuntu servers to reduce friction. This was true for me, I hated dealing with CentOS because of the minor incompatibilities with Ubuntu, so whenever I had the chance, I pushed/chose Ubuntu on the servers. So little by little, you hook a generation on Ubuntu.

It's a bit like the old Photoshop strategy, you let them pirate the software when young to hook them up, and then they would demand it professionally.