I'm sure a big part of that is that it's easier and less costly to the seller to unload cheaper low/mid-priced basses locally. No dealing with shipping, PayPal fees, insurance, etc. and the sub-$500 market is the majority of the market anyway. On the flip-side, selling a $1K bass on craigslist or your local facebook group can be a fools errand if you don't live in an area with a good music scene or a large number of people.
I honestly do the same with pedals a lot of the time. I can easily unload pedals from Boss, MXR, TC, etc. locally without much hassle. If I'm selling more expensive or boutique stuff, I just list it here where people know what the hell it is and what its worth...