> I'm sorry but why C#?

Because Stardew Valley is written in it. Probably half of modern games are written in C#, too (Unity). It's not exactly an odd choice.

> Isn't the big tech (Microsoft) in this case is a very big downside?

No, a language being backed by big tech is a plus as long as the stack is fully open-source (which .net now is).

> Why not LÖVE (Lua) for example?

Because Stardew Valley uses MonoGame, not LOVE.

> There is also libGDX (Java) but not sure Oracle is any better than Microsoft.

Other than Minecraft, can you name one successful game written in Java?