3rd party libraries don't spring from nowhere. And no language starts having one in abundance. People have to be motivated enough to write all those libraries in the first place and a lot of them are written just to use python syntax over C Code.
I'm not saying it's the only reason to choose python now but it's definitely among the biggest reasons.
Why not just call the C code you've already written in C ? Because they would rather use python (or python like) syntax.
I don't think we actually disagree here. Even your point about the better C-API doesn't indicate that syntax wasn't a deciding factor, just that one of several options had better compatibility.