It sounded to me like they weren't able to get enough revenue going for their work, and are basically throwing in the towel.
To me, it sounded as if they're hoping that their audience increases to a degree that the donations will finally be able to make their livelihood sustainable... Or not, which will make this a side gig
I sort-of agree. I don't think they're throwing in the towel completely, but clearly their current model isn't working, and they need to try something else, and they know that that might not work either.
Nonetheless: they want an alternative model that might make this sustainable, and I suspect they'd have better luck trying an open-source with a paid tier model, rather than pure donations.
I've never used the Sourcetrail, but I suspect there are some simpler and more advanced features involved. They could open source it as here, but put the advanced features behind a simple paywall. That would still give the benefits of open sourcing (more community engagement, wider usage of the tool itself) whilst being far more likely to drive the heavy/enterprise users to actually pay for it. Asking people nicely for donations is much less effective than asking for money to use a valuable feature.