I should clarify my thoughts. I'm unusually attuned to the bleakness of the realm, as well as the ornate vocabulary of CAS. I think they're spectacularly done, and wish such achievements of style were done more in literature. Of course, those themes are at a remove from what normal readers can stomach, hence the cautious note that I mentioned. On Veeky Forums it's unnecessary, but I felt like I had to do it.
HPL was the face of the weird fiction group, so when Lovecraft died and CAS's parents died (as well as REH), he had little reason to stay connected with Weird Tales. So yes, I do feel that it was essentially done to pay the bills, though CAS put obvious care into his fiction.
The lack of recognition, of his forelorn child poet prodigy status, and his poverty, contributed to his situation with Weird Tales. He had to sell stories that the kind of people who read Weird Tales would read, thus I can see why he was discouraged from enhancing his work with meditative cerebral depth. Lovecraft had a more stable financial situation, and could absorb the backlash he received, as with some of his quasi philosophical manifestos. CAS didn't have as much wiggle room. So, instead he reveled in the wounds from his youth, through his literature, with themes of abandonment and decay, without making the leap toward maturity. And while the death of a close friend tends to send other types into virtuoso overdrive, which might have enhanced the depth of his plot structures and themes, he never did.
It should also be noted that CAS also dabbled with painting and sculpture (and women); he had an artist's drive, which tends to be directed at beauty, and not loftier objectives. His painting and sculpture are simple and childish, not even approaching the skill of a practiced intermediate like Blake. They echo the simpleness of his plot structures. If he didn't take his fiction too seriously, then the question of him achieving broader success isn't as important as what his temperamental successors would accomplish.
CAS didn't really have the will to even attempt to make a breakthrough. With his ties to his life's situation via his parents and lack of money, the societal constraints against him, of institutional bias, and lack of widespread audience appeal, and his own uncertain drive in life, then maybe he didn't need to. His style is overbearingly drudgy for most readers, though I consider that their loss. And while CAS has been mostly ignored in his own lifetime, Vance came along and copied his style, without properly acknowledging him, and achieved mainstream success for him. Vance's success indicates the level of recognition CAS might have gained, had he continued his fiction career.