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.