Shows you how the independents and left aren't strictly anitgun. Was such a huge blunder on his part and the person asking him gave him repeated chances to walk it back and clarify it to not mean what he said, and he still insisted on total ban.
If I was a registered Democrat in Texas and actively didn't want Beto to win because of his anti-gun stances, why would I vote for him in the gubernatorial? I can't vote R, so best thing I can do is just not vote.
Biden is much less likely to succeed in pushing national level bans compared to Beto and state level bans, so voters who didn't vote for Beto might've later voted for Biden.
Also, presidential elections simply have a higher turnout.
Nearly 2 million people that voted for biden were already at the polls and didn't have the 5-10 seconds to put a check mark by Beto's name on the same ballot?