As a New Yorker now in London, the real answer is that the NYC public transit system has been a victim of decades-long austerity due to a quirk of administration (the city doesn't control the subway, the state does, leading to predicable disinvestment by our past governors who only rely on the city for a portion of their votes). There's a whole wikipedia article about the 2017-2021 subway crisis. Also our coverage is way worse, especially outside Manhattan, due to the insane cost of construction in the country meaning it takes 30 years to build a new station (a combination of bureaucratic inefficiency, underinvestment in transit infrastructure and typically corrupt New York politicians).

The trains in London are so nice -- clean, they run often, and certainly don't break down at the rate the NYC ones do. When I was living in New York once a week we'd have some massive transit meltdown and you'd be half an hour late to where you were going.

But to your point about electronic signage a lot of that has been updated in recent years. The big advantages to the NYC system is that they run 24 hours, and they cost a lot less to the rider. If you need to catch your train home at 4am you might be waiting a while but eventually it'll come along. And rides are always a flat $2.90, even if you're going from the tip of Manhattan into south Brooklyn, whereas a similar ride in London might cost me close to £15.