Prior to the whole Bike Cuck thing, it's because he was a webcomic artist with a modicum of talent that, by his own admission, threw out any and all forms of artistic integrity and creativity in favor of churning out the most mass appeal, generic "relatable" comics that have increasingly become the death of webcomics as people try to pander to the facebook and twitter crowds with repostable, inoffensive and generic schlock that had no message beyond "boy, life can be hard sometimes, amirite?"
He had the ability to make good comics, but realised that the comics he put effort and thought into got very little response, while the generic, phoned in "relatable" comics would get reposted everywhere with a bunch of likes. So he gave up on the former and instead went with the latter since, logically, low-effort comics that make a return on investment is preferable to high-effort comics that don't make a return on investment. If I was him, I'd probably have done the same thing to be honest, but as someone that cares about comics as an artistic medium (as, I would hope, most of /co/ does), this kind of mentality is pathetic and almost sickening. Choosing popularity over artistry is a slap in the face to anyone that cares about the medium.
It's like comparing Heathcliff and Garfield. One is a brilliant work of art that exists as an avenue for its creator to explore the idiosyncrasies of American culture through the lens of avant-garde americana as presented by the titular character. The other is "LOL MONDAYS & LASAGNA XD." Obviously the latter will be more popular because it's "relatable," but its still assembly line schlock with no artistic value whereas the former may have less wide appeal due to its niche nature, but fuck me if Gallagher isn't pouring his heart and soul into it.
After the Bike Cuck fiasco, he seemed to get a bit more self-awareness and is trying to put a bit more creativity into his comics though, so good on him.