The only reason the GOP is as powerful as it is is because the Dems are so fucking terrible at playing the damn game. Pick your battles. Good idea or not - Biden is trying real hard to lose this election.
The biggest single-issue voting blocks in the county are pro-lifers and pro-gun people. Even if most people want stronger gun control and better abortion access, they don’t base the entirety of their votes on those positions. It’s not like Dems or moderates who are anti-gun would vote for Trump or Biden were pro-gun.
The only time being pro gun-control is advantageous is in a primary, which Biden doesn’t have to worry about. In the general election it’s entirely detrimental to a campaign.