Just speculation, but if you are willing to boycott a business for being shitty, you likely aren't going to go out and be a shitty passenger yourself. I'm sure that isn't all of it, but I could see it having a partial effect.
I know some passengers (myself included) prefer Lyft because they do more driver vetting. For instance, to drive for Lyft you actually have to have someone from Lyft get in the car and ride around with you, which is not a requirement for Uber.
Perhaps the people who value that sort of thing are better behaved? Or more likely, it's probably because the people who are willing to pay slightly more for a better experience are better behaved (like Walmart vs Target).
Anecdata: Before I deleted Uber, I had two bad interactions with drivers (including one who threatened violence in response to I have no idea what, some misunderstanding). Since only using Lyft: zero.
That also stand in contrast to exactly one two experiences with cabbies over ~25 years.
I conclude, based on my limited personal experience, that vetting drivers makes a noticeable difference in driver quality.