Knowing many over my adult lifetime, and having BEEN a former Democrat, I can think of many :
1. One is - habit. Admit it, if you're Republican, you've cast votes for Republicans without really checking them out.
2. Another is - no choice. In the District or in Massachusetts, for example, the GOP is so very weak it sometimes - actually OFTEN - doesn't bother to even field a candidate at all. Your only choices at all occur in the primary. So you might as well register as Democrat, because the primary winner WILL win.
3. From MY personal reasons - until some time back - the Republicans by and large preserve the way things are. They're NOT innovative. They're at best reactionary. They would not have started a space program like we did in the 60's. Sometimes, LACK of innovation is good - right now, the Green New Deal is "innovative" but it also portends to be a massive, wasteful, corrupt boondoggle, because you can't promote a technology the public isn't interested in embracing. They LOVE smartphones - solar, not so much.
4. The press. You and I know it, but the press out there is EXTREMELY biased against the right, and every survey or metric used to measure bias has amply demonstrated this, repeatedly. Journalists are possbily the highest percent left-leaning of any demographic you can conjure. The press is still saying Trump colluded with Russians, that Hunter Biden is a victim or Russian misinformation, Sicknick was beaten to death with a fire extinguisher and there's no problem at the border or in Afghanistan.
5. Social media and pop culture. There actually does exist a portion of the population that pays so very little to what goes on in the world that they take at face value whatever the pop culture puts out there. Oddly enough, these aren't always dumb people - they just don't scrutinize the information. They don't want to.
6. Lastly - this one I don't know. Stigma? Perception that they are ALWAYS the "bad guys"? The whole "Walkaway" movement was born from the idea that the Dems DO NOT want to help you. Because they haven't been doing it. They pay lip service during elections, but at best toss bones to the constituents they keep in fear of Republicans.
Me - I think it is ridiculous to think one side is ALWAYS THE BAD GUYS. But the problem is largely, NATIONAL party politics. I was watching Justin Amash on This Week - (I still despise the guy) - but he mentioned that he was either surprised or bothered by the fact that he was sent to DC to represent his consttuents, but the PARTY makes sure you ignore that - and support the PARTY.
See, I am not in love with the GOP - they're part of the problem, too. The problem isn't that the OTHER guy is bad - the problem is, there's a strata of people running things that fight with each other over dominance - but none of them want to UNDO the power they all share. It's like decidiing which prison guard you favor while they wrangle over who gets to beat your ass or screw you. As Carlin once noted, you tend to favor the person you think will screw you less.