Dems successfully completed their main mission; take over left-wing movements (or at least groups with potential), take them over and turn them into "voteblue" movements, then destroy them. Sure, this isn't purely the doing of the DNC (some leaders of the BLM movement were killed by cops, some sold out, some walked away), but as soon as Trump became president - especially when he started to attack Kaepernick, this started to happen. Then on 2020 election day, Dems immediately started blaming the BLM/defund movement, even after they successfully turned it into a "vote blue no matter who" movement.
One thing for sure they do is get funneled a lot of money from regular people who want to donate and help out BLM and these donations of course hardly trickle down especially for mutual aid like keeping black moms housed and kids fed. If there was no national org I think it's safe to say every city with a BLM would be able to do so much more with they money that should be going to them.