Pay.
Pay.
At the same time, this attempt nicely illustrated that the chain is only as strong as the weakest link since, as I understand it, no part of the backdoor was committed to the git repository in cleartext. Instead, the part of the backdoor that was at least somewhat identifiable was only included in the tarballs that would be downloaded and used by Debian/Fedora when building the packages for these distributions, thus giving a very nice trade-off between the chance of someone detecting what was going on and the potential impact of the backdoor.
Depends upon your perspective.
Hacker: "Oh it was hilarious, you should have been there! They donated $1M to the project after I hacked the code, so I took the $1M too."
Time is another factor. It takes time to maintain software, improve the codebase, add features, etc. Then there are the other tasks such as answering questions, reviewing PRs, triaging bugs and feature requests, etc.
So getting more contributors, people to assist with bugs and bug investigations, etc. is arguably more important. Especially projects developed by a single person, or a small number of people. That's the avenue that opened up this attack.
It is easy to get burned out implementing features that end up being more complex than expected, interacting with users that want different things from a project, and having a growing list of issues and PRs. That's the scenario that happened with xz, and is common with popular software that is maintained by a solo developer.
The other aspect to this is the direction the maintainer wants to take the project in. If another maintainer has a different direction in mind, that's going to cause tension.
Time and money are not actually 100% fungible, but there is a lot of truth to it, especially given enough money.
Maintainers are human. They need to eat, to sleep, to visit the doctor, to rest when they get sick, to participate in activities that reduce stress and foster human relationships. Money makes all of that much easier.
Pay is how you get more contributors.
Besides, the maintainer in this case was already taking time off regularly, not to work on xz, but to get away entirely from any kind of programming work. Throwing money in his general direction probably wouldn't have helped with the burnout, unless you were offering to help him hire somebody.
Of course if the developers don't want to be paid, then that's that. But otherwise, there is a very heavy atmosphere in the open source community of excommunicating anyone who dares to ask for payment as heathens of the vilest order.
I fully agree that forcing payment or using dual licensing is unfortunately heavily frowned upon. But a voluntary Patreon/donation option is perfectly acceptable to the same anti-payment people.
But it's not a solution for all problems.