We were concerned about finances and legal protection.
1. The project would deal with user's data to some degree
2. The project was going to "annoy" an existing, much larger, project who would have likely tried to take some legal action to keep their "place at the top"
3. The project was going to both a) need to generate funds (and pay core developers), and b) be guaranteed to generate funds, based on our experience. However, we did not want to register a company as not having a company complicate things was one of the central goals of the split from the larger project. Try paying people a couple hundred bucks (less than minimum wage, more like Aufwandsentschaedigung) without having to jump through various hoops and without doing it illegally.
There are about 100 categories that are predeclared as potentially charitable and you have to fit into one of them. Most of them are weirdly specific like homing-pigeon breeding or model plane flying.
The only two that are broader and remotely realistic candidates for a FOSS project are religion and education.
If you don't want to start a cult you are left with education. That is as how organizations like Chaos Computer Club do it. Education means education for the general public though and it is not enough if you offer occasional courses for a niche topic. It has to be something that potentially interests everyone. The tax office is checking that and it is on you proof it to them regularly.
Chaos Computer Club is not a charity.
I believe the "Chaos Computer Club München e.V" is (or was at some point in time) charitable. At least their bylaws have the magic words:
"Durch die genannten Zwecke sollen Kultur, Bildung und Wissenschaft gefördert werden."