Is it also rude to ask you about your hobbies?

Asking why seems like a polite enough question to me. They could just as soon ignore the request.

It sounds like people are potentially open to the idea of a language exchange, but want some convincing that you're serious about it or the right fit.

Not to be rude myself, but why don't you just change your approach then, and say why in your initial message, since there seems to be a pattern here. I'm not saying you should tell them your life story right off the bat, but your reasons aren't a big secret either, right?

And believe it or not, some people are initially incredulous that you even want to try learning their language, especially if they don't think of it as that popular.

Fair enough guys

It’s not rude to ask a potential language partner what motivates them to learn their target language. Someone might be looking for a language partner with similar motivations. There’s no need to beat around the bush about it and wait till you have a session together. If I’m incompatible with someone, I’d want to know as soon as possible.

You have no idea what wierd first massenges I get here as a women. (Mostly from guys who don't even speak a language I want to learn.)

I don't think it's super rude to ask why you want to learn a certain language, but maybe just an easy conversation start. It's the first topic you know, that you have in commen with the other person: Learning a new language.