You can enforce by requiring an agreement, and making the terms reasonable enough that an OSS project can agree to them (this isn't entirely easy, but it's not that hard either).

Also, there's the thing where these other projects have been using the name since forever, so there's already a history of non-enforcement.

The best time to start enforcing their trademark (from a legal perspective) was ten years ago. The second best time is now. Negotiating many agreements with many OSS projects is a lot of legal work which they may not want to pay for. And none of this undermines my original point that their primary motivation is probably just trademark preservation. That they did a poor job previously doesn’t mean they aren’t trying to do better now, and that they could use less aggressive methods doesn’t mean they aren’t primarily focused on trademark preservation. It’s the simple explanation. Occam’s razor.