I’m not very familiar with the grassroots, so maybe I’m way off.
I think some of the big effective animal adocacy groups started as grass roots, and then because they were judged to be cost-effective, they were recommended by ACE or funded by Open Phil until they became big and weren’t really grassroots anymore.
Maybe it’s primarily because big funders don’t value a lot of grassroots work (rightly or wrongly), and if they did, those orgs would professionalize and scale up.
Or, some grassroots work is necessarily too low-scale (even if cost-effective) and it’s not worth the effort to try to estimate its value. So, projects with greater scale will disproportionately be more well-funded.
Or, maybe grassroots work, even on the same things in different regions, is more variable in cost-effectiveness because of less structure and different organizers in each region, or funders expect it to be. A fur campaign succeeding in one city might not tell you much about whether a fur campaign in another city will succeed if they share none of the same organizers.
The Humane League started as a grassroots group, has a large network of campus activists so still does grassroots work, and they support smaller groups with the Open Wing Alliance. I think they have done training for other groups, too. Maybe they’re pretty unique this way, though?