If you start with a small transit network, and build it out over time, it gets larger and more efficient, providing more access over time through the network effect. But at some point, the dynamics switch and the operator realizes that they can get more money out of running more service on the existing service than on further expanding the existing network. (Which is financially prudent). But it breaks an implicit political bargain between city and suburb, viz: "If you pay to build ours now, we will pay to build yours later". As long as those network expansions are promised, and voters think they might be able to use them (or make use of them or benefit from their use), they'll fund them.
But if you are an exurban commuter, you'll never see any direct benefit, and you know it, but you are still paying for it. Which means sales taxes work basically in urban areas, where they can match the beneficiaries with the payers. People who drive into the county and buy things matter less politically, because they don't vote on referendums--but still do matter, as they still do lobby for things, as NJ and NY demonstrate with the congestion charge.
Anyway, if the transit network stops expanding, and the transit constituency ceases to include those folks, the transit providers switches to concentrating on existing riders. And then all the capital and operating costs go into providing additional mobility (more tracks, faster loading at stations, more trains/hour). And the main trouble, I think, is that transit agencies that have made that transition once have a hard time switching back to the other regime. Once you've quit building subways, it takes herculean efforts to get things like CrossRail and the Second Avenue Subway built. On the flip side, LA did manage to get the Regional Connector done.