Reader didn't have enough active employees to keep it running. Nobody inside Google worked on it any more, so it was just a zombie product running on inertia.
Reader's active users, though, tended to be influencers who read a lot and shared what they read with their friends on social media. So they still drove traffic, just by a different mechanism.
Many of the people I followed on Reader moved to Twitter after Google took it down and mimicked its social functions there. I'd wager they're still either using RSS readers or jury-rigging Twitter into being something like it.