Julian worked hard on the music, but was truly awful at promoting it.

While other members were so much more open to doing what it takes to get a platform to showcase their music, Julian was lecturing and insulting journalists, showed up as a non-functioning drunk, didn't want to show up to awards when it was not exactly on his own terms, recused top producers, he even punched tv producers who wanted the Strokes on their show.

He had a terrible reputation in the business and it really hurt every chance The Strokes had to become mainstream.
There's a reason why The Strokes who are objectively better than most other bands didn't get their career.

They were handed so many opportunities by their label: they got the advertising, their music was featured in ads, they were featured on Guitar Hero, they got popular actresses in their videos shot by famous directros (Julian turned a lot of them down, though), radios were payed to play them, fashion labels contacted them all the time and landed them clothes.

And you know, some of the members were tabloid fodders too, getting them even more opportunities.

But when the lead singer, the face of the band, refuses to do interviews, refuses to pose for magazine pictures, sabotage every opportunities with his behavior, the general audience is going to favor the ones who give themselves 100% and gives great shows, have great videos, give back to fans.

Julian today does a lot more in that respect than he used to.
Ryan did initiate fanzines, showed up on fansites, etc.
But there's nothing one can do when the lead singer acts lazy and uninvolved.