Depends on what you call "fundamental reason".

Building new mass transport systems like light rail in existing cities from scratch requires a lot of political momentum, basically willingness to crush the pervasive NIMBY mentality and overcome the pull of bureaucratic inertia. Even in European cities that never dismantled their light rail systems, inhabitants often fight back against line extensions, citing noise concerns etc., and are able to delay the construction for years or decades.

Plus such systems aren't cheap and usually require new subsidies on top of existing subsidies. You will still have to maintain the existing road system and bridges, their usage won't drop to zero or even a quarter of current traffic. Some people won't voluntarily switch to public transit ever, for all kinds of reasons.

Maybe this isn't "fundamental", but IMHO the hurdles to overcome are really high and I am not sure if there is any city in the US willing to try this and having the means to do so.