Yes, it's because the structure of the existing codebase for the typescript compiler matched go better. (There's a blogpost about it somewhere)

edit: I think it was considered, but go was a better fit

What does "structure" even mean ? How or why are models so much easier in Go compared to other languages ?

Go models aren't easier, but the existing TypeScript codebase leaned heavily toward structural types, which made it a better match for Go and at least initially made porting easier for them.

Yes, Anders Hejlsberg did various times in podcasts when they announced the move go. It has nothing to do with C# not being capable enough, but rather that a line-by-line port from Typescript to Go would be easier. I think IIRC several time that had LLMs be more advanced as they are today, that choice would have been a different one.