It is because the Democrats have no conscience. They might get behind the scenes and argue privately among themselves but when they walk out to vote, it is in lockstep.
Basically, they are good players at the game of politics. Everyone knows that politics is a dirty game, and they are simply good at it.
Republicans argue among themselves behind closed doors also. They then bring those arguments into the public. They are not shy about airing their arguments and therefore not as good as playing their dirty game as the Democrats.
I saw a poll a couple of years ago that had Nancy Pelosi nationally as the most hated politician in America. She had about 27% support and that’s even among Democrats. But she was the Speaker and when the Democrats went to the floor to vote, they typically did so as one. When they made public comments, it was as one. They are simply better at the dirty game.
As an example, nationally, the Democrats are absolutely opposed to private gun rights and reducing taxes. They do everything in the courts and votes to try to keep people from possessing firearms and to keep more money coming in. You know however, that the democratic rank and file voters like the people reading this forum, absolutely want to have the ability to purchase whatever firearm they wish and want less taxes out of their paychecks. You will see such comments as (even in this forum), no one is trying to take your guns. That is absolute nonsense, and you can look across the nation and see evidence of it time and time again but it will be denied. So you have the Democrat blue-collar worker who is against much of what the Democratic Party stands nationally or certainly how the party votes in the legislature but they were going to the voting booth and pull the D lever to support the home team.
They are better at the game.