Can’t say for sure, but I’d wager it’s because of campaign finance. Corpos fund campaigns and a pro worker 3rd party would be inherently against corporate interests. Anyone who tried to break away from the democrats would end up without any funds and new democrats would run against them with vastly more money.
It’s also worth considering that they’re probably not that popular. Most of the population are disengaged from politics and tend to just vote with the people in their communities. Text based social media tends towards a leftist bias and probably makes them seem more popular than they really are.
The tag I played melee under was “frenchfrymaster” which was stupid. So I changed over to my in-game nickname “FRYD” which was supposed to be “fried”, but a few years later I found out it’s the norwegian and danish word for joy and I started using a bastardization of their pronunciation.