[nerdix]

More important than being open source was the fact that it was Windows only for the first 15-ish years of its existence.

Mono never got traction within the startup culture. In part, because it operated in a legal grey area for many years regarding MS patents for web (or web adjacent) technologies like ASP.NET and ADO.NET.

[WorldMaker]

Microsoft made some extremely generous patent grants to Mono and several major Linux Distributions relatively early in Mono's history. Microsoft didn't seem to want to repeat the mistakes that led to their divorce with Java and the early foundations of .NET with their relationship with Mono. In my experience of it, most of the "legal grey area" concerns around Mono for a lot of its history was either truly outdated/ignorant advice or intentional FUD from antagonists (many of which just hated anything Microsoft indirectly touches no matter what).