The Republicans deliberately built the grassroots top down, with targeted media and subsidised networking. Supposedly grassroots orgs were aggressively seeded and astroturfed.

It's not that progressives don't pay attention, it's that the Democratic machine doesn't do this. In fact it works to sabotage and disempower effective grassroots opposition to its corporatist policies.

There are always a few break outs in every cycle - like Cori Bush this year - but they're there for show, not for policy. If they get too pushy they run into opposition, and if they don't accept a compromise with the party line they're quietly removed by PACs that funnel non-grassroots special-interest money to challengers.