The biggest reason is: J&J's biggest advantage early on was how much cheaper it is. One shot, simpler manufacturing process (which they've screwed up several times anyway, granted), it would be the one to get widely distributed because the cost-per-fully-vaccinated individual is far lower.
The issue is, the government is paying for all this, and they write blank checks. CVS administers a Pfizer/Moderna; insurance or the government refunds (I'm told) $50. They administer a J&J, they get (again, hearsay) $10. Vaccination clinics are actively incentivized to stock the most expensive vaccines, because the profit per vaccine is higher. So, some have stopped stocking J&J (which, coincidentally, is the one vaccine I've heard many anti-vax people say they'd actually consider, as its kinda-sorta-not-really-but-good-enough more traditional. at least one person i've talked to has said 'well, i'd get the J&J, but the closest clinic that has it is 30 minutes away, so i just haven't gotten around to it').
