This is where you have to remember how casually engaged most voters still are at this point. Biden was given frontrunner status on a silver platter and has proceeded to do nothing with it but slam a car door on his dick repeatedly for months. You'd think he'd practically be at bottom feeder status by now, but consider how little thought the primary electorate put into making him the frontrunner in the first place, and also consider that they aren't diligently following the day-to-day of the field like we do. A lot of people are sticking with Biden for now because they haven't noticed much of a problem yet.
From what I can tell, there are 3 things that voters perceive about Biden which make him popular:
1. He's Obama's sidekick and they miss Obama.
2. He's folksy and can be more appealing than Clinton was in the states that Trump flipped red last time.
3. He's "tough" and can take on Trump, without being the kind of aloof technocratic wonk that Trump dispatches easily with a childish insult.
Honorable mention: Some people actually think that because he was buddies with a bunch of Republican Senators back in the day that he will be able to put a stop to the extreme partisanship in DC. While I find the 3 aforementioned perceptions about Biden to all be questionable, this one is just stupid as f**k and if you believe it to be true then you have brainworms.