EAGLE is the tool they know. All their files are already in EAGLE's format, and even if KiCAD can "mostly" do import, that's not quite a matter of being able to pick up your design and keep on going.
In the electronics world, tool lock-in is a very real, strong, and painful force.
I don't have pity for people who choose to lock themselves in to tools and then ask for $800 because of it. Oh well, their choice! It really isn't much work to switch to KiCAD.
In the electronics world, tool lock-in is a very real, strong, and painful force.