Earlier quoted context omitted.
Platform support and open-sourcedness. The Phoenix 1.0 release predates the first open-source and Linux-supported .NET release by a year, for example. .NET is just now starting to shake off its association as a closed-source, Windows-only thing.
.NET has been open source since a decade
https://news.ycombinator.com/item?id=10135825
https://github.com/dotnet/core/blob/main/release-notes%2F1.0...
People aren't just going to jump onto something recently open-sourced by a company that popularized the phrase "embrace, extend, extinguish". For the last decade, they've had to earn people's goodwill, while with a project like Rails, there is no "we used to be closed source but now we're not, use our thing!" to overcome. So in the 20 years since Rails has been released, it has only ever needed to demonstrate its usefulness.
Now that a decade has past, that negative association is starting to wash away a little.