NEOs can have much lower deltav but have much worse Synodic periods. That's because the orbital periods are similar, and they take longer to line up each time. And for a near Hohmann transfer which most travel is likely to use, they have to line up.
So opportunities to travel there or back are few and far between. This makes them surprisingly useless as a propellant source, because unless the mining operation is unreasonably quick to perform, for orbital mechanics reasons you have to wait for multiple synodic periods before you return anything, and another one again before you can do it again. For example, if the synodic period is 5 years it could take 15-20 years to get your first shipment.
Ceres is further out, but its synodic period is only 466 days; so you end up with a propellant shipment every 466 days.
Getting to Ceres initially with mining gear is harder but a penalty that you only have to pay once, and you can use ion drives which can have high exhaust and deltav for that, but once you're returning propellant, you have the propellant you need to send further stuff to Ceres from Earth, and Earth to Ceres, so, although the deltav is the same each time, the effects of the delta-v penalty aren't quite so severe and you can set up cyclers to make the trip repeatedly at lower delta-v, and use aerobraking at Earth for the propellant.
The other thing is that Ceres is outside the snow line; most NEOs have probably been baked out of volatiles on their surface, so mining them is much harder. Ceres is further out, so ice evaporates very slowly.
All in all, Ceres looks like a much better bet all round.