Paste Magazine wrote an article attacking this very same question from the opposite perspective back in July, when that Attack on Titan 2.5 million number hit.
Two parallel mainstreams seem to have developed. One mainstream includes Marvel, DC (and very soon Image) and the other entails books that actually dominate the sales charts; in other words, the comics considered mainstream by comic readers and comics considered mainstream by statistics. This dissonance is best illustrated thus: much of the general public still thinks comics are for kids (which is why every mainstream publication has published a “Comics aren’t just for kids!” headline at some point); primarily, comics reading circles maintain that the medium is designed for adults—and not kids, which is how you get a DC comic where Frankenstein sews Black Canary’s head to his chest.
Marvel and DC are consistently considered the mainstream, but they are routinely beat out in pure sales and popularity by webcomics, manga and graphic novels including Telgemeier’s Smile and Drama (both with more copies in print than Star Wars #1, and at a higher retail price). Homestuck is a webcomic that was, at least at one point, getting 600,000 unique visitors per day, and Randall Munroe’s XKCD was reportedly pulling in between 60 and 70 million hits a month at times. The last pamphlet comic prior to Star Wars #1 to sell one million copies was a Pokemon manga from 1999.
Ironically, Marvel’s own Ms. Marvel presents a perfect example of this scenario: the ongoing title remains one of Marvel’s most successful series, but its success is still qualified and written off, because it’s just “placating a vocal minority at the expense of the rest of the paying audience.” Another prominent example is how The Walking Dead #100 sales were qualified—it sold nearly 400,000 copies, and the series’ collections sell consistently well in bookstores and online, but it’s still an “indie” book despite evidence to the contrary. Book after book continues to regularly dwarf the sales of Marvel and DC output—some even casting a shadow on the historic sales of Star Wars #1—but their success is overlooked. Regardless of how many units are sold, Marvel and DC are mainstream and everything else is “other.”