One reason might be that the tools have telemetry (i.e., automatically gathering and sending data about your system and your use of it to Microsoft) enabled by default[1], which is unexpected and off-putting to people in the free software world. It seems to reflect a very different attitude to some fundamental cultural values.

According to the comments on the issue tracker, it was supposed to be OK because it was anonymized – but oops, there was a bug, so it wasn't totally anonymized. But it's OK because you can disable it if you happen to know about it – but oops, the disabling mechanism had a bug.

Things that are important to many free software users are not important to Microsoft. I was excited about .NET Core until I became aware of this stark misalignment of goals and priorities.

[1] https://github.com/dotnet/cli/issues/3093

Mainly historical ones I think. It's been open source for 4 years, but for most of that time there was a paralell closed source implementation which complicated the ecosystem a fair bit.

I'd say it's only really in this last year that it's really started escaping its legacy and started really making sense as a non-microsoft stack.

It's open source, but does it comply with the four freedoms to make it FOSS? Why does mono still exist then?

inb4 "but EEE was only the 90s!"

Mono still exists for the same reason .NET 4.X still receives updates, amongst other things. It also has better mobile and crossplat support for targets outside of the server realm