Trademarks cost serious upfront money - Germany alone is 290€ [1], EU-wide 850€ [2], the US 250$ [3] and worldwide is a hot mess [4]. Additionally, it seems like you need a lawyer in the US to handle the process if you're not an US resident, and you have to renew them every couple of years and not forget to tell the patent/trademark office of address changes. Not everyone is willing to put up that much money and effort for an open-source project that won't generate any income.

Additionally, trademarks usually force the holder to publicly register their name and address, which is simply not a good idea at all for many persons - trolls, spammers and outright criminals can and will use every bit of information they get to cause harm. And for collectives that run an open-source project, trademarks will be yet another issue - what to do when the holder of the trademark dies or disappears, or when they get into some sort of conflict?

[1] https://www.dpma.de/marken/

[2] https://euipo.europa.eu/ohimportal/en/fees-and-payments

[3] https://www.uspto.gov/trademarks/basics/how-much-does-it-cos...

[3] https://www.wipo.int/madrid/en/

Software licenses need to work in a lot of different legislations, and it’s really hard to make a license that means the same thing in all countries.