DNF 3: better performance and a move to C++

I expect that there are multiple reasons for not being considered. I guess some of these are:
- The languages are very different. They'd have to fully rewrite entire chunks instead of a more gradual migration.
- The use of garbage collection might be a reason why they switched away from Python. A good garbage collector often exhibits a higher baseline RSS, which might be a problem on resource constrained systems.
- Go <-> C interop isn't considered to be very efficient. So, depending on which parts they'd want to rewrite first, there is a good chance that performance will suffer before becoming better.
- Upstream Go has (very) limited platform support. Gccgo can be used to work around this, but that would mean a workaround for a number of primary supported platforms.

And there are of course the philosophical differences, like to C++ gives you things like a rich type system, templating and exception based error handling. Go goes almost completely the other direction: simpeler type system, no templating, no metaprogramming and written out error handling. Both have their pros and cons.

But the project was probably looking for a way to incrementally make the current code base less cumbersome/easier. Switching to Go would not be the quickest way to get there.