The same reason supersonic passenger jets are rare.

The extra speed comes with a massive increase in costs.

Travel 30% faster than high speed rail for 10-20 times the cost.

They're super expensive. Few people are willing to pay the massive amount extra for the slight dectease in travel time. Investors also know that.

What is a Maglev train? (From WIki)

Maglev (derived from magnetic levitation) is a system of train transportation that uses two sets of electromagnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage of the lack of friction. Such trains rise approximately 10 centimetres (4 in) off the track. There are both high-speed, intercity maglev systems (over 400 kilometres per hour or 250 miles per hour), and low-speed, urban maglev systems (80–200 kilometres per hour or 50–124 miles per hour) under development and being built.

Why so little?

Despite over a century of research and development, there are only six operational maglev trains today — three in China, two in South Korea, and one in Japan. Maglev can be hard to economically justify for certain locations, however it has notable benefits over conventional railway systems, which includes lower operating and maintenance costs (with zero rolling friction its parts do not wear out quickly and hence less need to replace parts often), significantly lower odds of derailment (due to its design), an extremely quiet and smooth ride for passengers, little to no air pollution, and the railcars can be built wider and make it more comfortable and spacious for passengers.

Cute link to the Wikipedia Page

I am a curious human, beep boop

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.

I read it's the zoning and maintenance of the tracks. Since they probably have to be very precisely laid in order to support such a fast train.

That said I do wish for maglev trains to be accessible to all. I'd love to go across the states in a few hours on a train.

You still need rubber wheels when it's stopped and at low speed. They retract when it's fast enough for the maglev to take over.

The electrical conductors are expensive as shit. The ones in the train need to be super cooled or something. The track ones need to be built along the entire length. On three sides, one vertically and two horizontally. Along with massive power lines along the whole length. They don't need to move to be expensive.

The right of way needs to be very straight. So compared to normal high speed, you have to spend much more on buying land, earth moving, tunneling, etc.

All this needs to be maintained to an extremely high degree because you can't accept a failure. The engine on a high speed rail fails and you just slow down, no biggie. HSR track is fairly robust and can easily be inspected visually. Since it has the same base as normal passenger and freight you have an entire industry knowledge and inspection machines. Any part of maglev fails and you have a catastrophic failure.

One other thing I've not seen mentioned yet is capacity. Switching a maglev track is difficult and very slow, which reduces the number of trains you can get through a switch and therefore the number of people your system can carry.

because cars cars cars

Here's an interesting write-up about an attempt to develop a large-scale urban maglev system in the 1970s: https://en.wikipedia.org/wiki/Krauss-Maffei_Transurban

tl;dr: there were so many technical issues that when the West German company developing the tech lost funding and the Ontario government took over the project, they immediately abandoned the maglev concept and replaced it with linear-induction propulsion with steel wheels on rails (the mag, without the lev).

Even this tech, which does have a few advantages over conventional rail and is still used today in cities like Vancouver, is falling out of favour due to general logistical issues with using bespoke technology over conventional rail -- fewer people know how to build and maintain it, you're relying on usually just one company to supply your trains and infrastructure until the end of time, you can't reuse any existing infrastructure, etc. I'd imagine these issues still get in the way of maglev development today -- even more so because you can't even reuse existing rails

It makes more economic sense to improve the rails we already have, and build faster trains to run on the existing rails (like the TGV), than building completely new infrastructure.

I remembered seeing a video by Real Engineering that explained a lot on Maglev and it's pros and cons but one of the summaries that really hit it off for me and if I remember correctly is that it cost 11 times more to build per kilometre compared to conventional high speed rail, for about 70% more top speed while using 30% more energy.

https://www.youtube.com/watch?v=S4L_0CDsd1I

Personally I feel unless they come up with better superconductors, there's still a long way to go before it really takes off.