It's easy. Facebook already had the users. Nobody had to sign up for an account to sell on marketplace. You just go on, post shitty pics, add a shitty description.
It sucks because Marketplace is such a piece of shit. The search doesn't work, it always interjects shit that you don't want. It doesn't find all items within certain search parameters, etc. About 1/20 times that I do a search, the entire page comes back blank, or with 30 sponsored listings and no actual results.
It's the same reason that Facebook Groups took over for a lot of forums. Using a forum took some effort. You had to create an account, log in, upload photos to a host. You had to be really into it... and so a lot the low-quality shit was filtered out in the forums, just by being sort of hard to use. Now you just go on FB Groups make a shitty post with some shitty pics, no effort, thought or time has to be put in. Flooded with low-quality crap and repeat questions and topics.
You go on the groups now, and the quantity of the same questions being asked over and over is ridiculous. The search sucks, Facebook Groups are trash.
This is just like how Microsoft Teams came to be. It didn't get popular because it was a good product. It's literally one of the worst software applications that I have ever used. They already had the userbase (all the O365 subs), and gave it away. People take shitty and 'free' over good and costs money. Competition crushed.