Probably because Biden supporters are less 'online' and there aren't that many dedicated supporters of his (as in there are more people who simply think biden is better than the alternative than actually liking him). Contrasted to a personality like Trump who amassed an online dedicated cult following, in which they would literally try to decipher every word he said for hidden messages from the God emperor himself.

In the same way that Twitter seems to be full of anti-capitalist anarchist communists, demographics of the real world turn out to be a lot different than social media makes it seem.