story
We attempted a move to mono for backend web services maybe 3 years before .NET Core released, and it was a complete no-go. 10x reduction in performance on the same hardware.
This wasn't specific to our workload either. I was big into the game Terraria at the time, and I saw similarly poor performance when running it's game server under mono vs .NET 4.x
While some of the mono toolchain was integrated into .NET Core, CoreCLR was a rewrite and immediately solved this problem.