Where existing transit infrastructure exists, cities prefer upgrading existing infrastructure, rather than installing new infrastructure in its place, and where transit does not exist cities prefer not to install anything at all and favor cars typically. Maglev trains are extremely expensive to install the infrastructure, so gathering the money out of local budgets to invest in the extremely expensive maglev infrastructure is typically very difficult.
In the US in particular, politicians, just don’t look at the picture in the long term, and only focus on short term investigator as it pertains to their election schedule, and that is sad and has long-term impact on the local population.
Think about it this way, OP: You know when they're working on the train network, how much you loathe commuting while a single line is out? How much of a pain replacement bus transportation is?
Now imagine having to do this for all train lines, everywhere, and you always have to switch trains (due to the difference in track) in between the blocks of replaced track. Plus you can't neatly fit maglev where conventional track fits and vice versa, plus you need the power infrastructure, plus you need to find a way to buy the rolling stock without already selling the old one.
Also for the US the automotive and oil industries have powerful lobbies and an obvious interest in preventing the proliferation of electric-powered public transport. They’ve spent decades centering personal automobiles as the default method of travel and attack these projects with enthusiasm.