[usrusr]

Lack of network effect, perhaps compounded by inadequate routing (I have no idea if they were as good as possible or much worse than necessary).

Network effect must be huge for a system like that. Imagine a large fraction of the cars on the road was shared in the way of supershuttle, you'd already have in that pool a near-perfect itinerary to tie into for almost any trip. And the remainder could easily be fulfilled by assigning a new trip. If you just have a few cars and price for shared occupancy tours will inevitably be much worse. But once you have a critical mass network, route inefficiency will be just a load factor price/performance tradeoff like in a hash-map.