It was definitely the fact it was Windows-first, IIS-first and smelled of Enterprise Software (i.e. Not Cool(tm).) mono was workable but second tier, though I feel it did help convince MS that the dotnet project is worth pursuing.
Today I'm running a home asisstant with a side service written in dotnet on a raspberry pi 4 raspbian in docker containers. If it didn't advertise the dotnet version in the logs I'd never know.
No drama over Generics, Exceptions, Lambdas/Closures, functional vs procedural syntax, etc. C# either did a lot of this well already or was quietly added to the language in broadly sensible ways. The package management isn't perfect, but I don't think any ecosystem has mastered that yet. It's more sensible than NPM at least. ( But what isn't! )
And all the while, a lot of work has been done for performance, where now I'd trust it to be as fast as almost anything else. I'm sure the SIMD wielding C/Rust experts can out-perform it, but for every day code C# is writeable, readable and still performs well.
Okay, so C# 1.0 was a clunky beast, it wasn't until version 2 where they fixed things like captures of variables in closures when it became pleasant to write. I don't know if it was Microsoft's ownership or the 1.x C# experience that put people off.