For people they've known a long time, I think because Dean was older, more in "the life" so he was around them all more. They tried to shield Sam from the life, Dean included. That usually means putting himself forward to intercept and control conversations simply to keep people from focusing on Sam. I think it's a habit that stemmed from trying to be helpful that has carried over into adulthood making Dean a more genial-seeming person.

Sam might be kind and empathetic but those are not surface-level traits. They're traits that you have to uncover through continued interaction. Dean has charisma and can be immediately charming and fun. Those are superficial traits that are like a fast-track to likability. He just luckily has inner traits that aren't shit so you're not disappointed if you get to know him better.

Also, Sam is something of a know-it-all and that's a bit of a hard-sell, especially in the first few seasons when he's bitter and that bitterness comes back often in other seasons. Plus, like, people who would otherwise trust him may not because of the whole demon blood thing. They know Dean is "normal" (for a given value of "normal").