Probably just resources. Mozilla has to vet each update to give them the "recommended" badge and so they probably focus on the most popular extensions
> Violentmonkey - Rating 4.7 (793 reviews) - 164,622 Users
On Google Web Store, it's 11,000,000 users VS 900,000 users.
Their popularity seems different by a magnitude, hardly surprising Firefox/Mozilla would chose to focus on one above the other.