Ubuntu and Debian are, believe it or not, rather selective. .NET Core isn't "stable" enough for them.
To be fair, it has long way to go to reach the tooling, testing and maturity the JVM has in linux.
This is kinda what I was alluding to. I don't think it's quite ready for primetime. That said, I wouldn't be against seeing it in, say, Fedora's bleeding edge repos.
Is Mono ready for the prime time? Because it's most certainly already in Debian. It's just that MS doesn't give a shit about adoption on the "cancer" platform. They'd much rather we all wore the Windows ball and chain.
So you're one of _those_. Noted.
PPA is specifically designed for packages that are potentially not stable. And you can find dozens of garbage packages in Debian repo without even trying all that hard.