One obvious one for me, being an east coaster transplanted to the Bay Area: most people don't want to share a ride with a stranger. Bay Area (and perhaps the rest of the Northwest) upper middle class people are somewhat different. A more extroverted culture.
Zimride (where afaik Lyft got started) was all about longer distance p2p rideshares, and I was a driver and passenger on rides between SF and LA a bunch of times through that site before Megabus started up again.
As a driver, I got as many as 10 viable requests per seat regardless of my ride schedule. I saw a lot of empty seats in other SF-LA rides so I was confused at the disparity...until one ride, a college-aged woman that bought a seat showed up with her mother. Both of them expressed visible relief at the fact that I, a woman, was driving, and with two adults that were obviously my parents in the back seat just like my blurb said. They really were just going to walk away if anything was different, and they didn't even consider male drivers.
It clicked in my head that most messages were from women that must have felt safer riding with another woman. I naïvely assumed all the happy riders were just happy about a fast 5-6 hour ride with fresh fruit and snacks from my parents.
tl;dr I think it's about intimacy. Even for an extroverted culture, plenty of people didn't use Zimride and similar even if they knew about it. Sharing 1-6 seats (most private cars) is way more intimate than 6-10 (airport shuttle-types) or a bus/train and is typically not backed by a company with a professional driver. Not sure that there will ever be a solution that provides enough trust and support to get over that problem before we get better mass transit and self-driving cars.
And my friend made https://github.com/thedjpetersen/ridezap_os
And I've seen countless school projects trying to address ride-sharing.
I don't think it will ever catch on.