there are few maglev manufacturers, allowing vendor locking and exacerbating the first point
they must be built grade-separate, which can complicate route planning
they are incompatible with existing rail tech, which results in having to build new, expensive infrastructure for 100% of your route, further exacerbating the first point
their switches are slow, limiting capacity
Ultimately, their competition is regular trains, which are simpler, more tolerant to buying from multiple manufacturers, still significantly more efficient and faster than anything roadborne, able to switch over the course of seconds instead of minutes, able to interoperate with different tiers of intensity and speed, able to be built at grade, cheaper and having the better part of two hundred years of technological refinement behind it. Ultimately, maglev has specific, niche advantages that make it a hard sell for any system that already has regular rail.
On one side, it can mean this. And for lighter railway construction, such as trams, light rail and rural regional trains, this can be optional. Plus it makes yards and depots easier to build, just slap a few concrete plates for a few crossings and the staff will find their way around. This is not possible with some maglev technologies
On another, it can also mean that the infrastructure is built directly on the ground. Being able to do so is extremely useful, since you don't need to build (as many) bridges or tunnels to have rail going somewhere. Again, for some maglev technologies, this is not an option.
A lot of these arguments apply to high speed train. In France a completely separate line was build between Paris and Marseille for the TGV To reach its peak speed without being delayed by lines that stop at every station.
The problem is investment and shitty companies holding these technologies IMO.
To compare our bullet points for maglev and high speed & conventional rail:
there's a big step in price between "a railway line built to millimetre precision" and "a completely new type of infrastructure that may or may not need superconductors to work"
there are way more manufacturers for components of high speed rail. For rolling stock there 4 in Europe alone, plus more in Japan, South Korea and China. As for signalling, it depends on the underlying tech. And if we follow the current European tech standard, that encompasses a standard... That is made by multiple manufacturers, and where their systems are operationally compatible.
for one, high speed rail is still rail, so building at grade & laying them on the ground is trivial compared to maglev systems. While some maglev technologies must be built like a bit of a monorail, which must be built elevated everywhere. And if we step outside of high speed rail, and point to rail in general, the mere existence of level crossings and street running disproves the fact it can only be built grade-separate. Sure, level crossings for HSR are a reason for the planning engineer to get fired, but for rail in general, it can happen.
this depends on what is already there. Most high speed trains use standard gauge, and those that don't, use broad gauge, and for that, only the Russian Sapsan comes to mind. If your country has standard gauge track, you can use the existing railway lines into town, disproving your point. And as for signalling, you can fit a train with multiple train protection systems, and many places are instead working towards using high speed signalling for general use, since the tech that makes high speed trains go BRR is the same tech that allows regular trains to run closer together. Finally, electrically, many countries use the same power on the wires on their high speed network as their general network.
I've seen footage of a set of switch points with a diverging speed of 160 km/h, and it needs 9 switch motors to work. Proper high speed switches need even more, last time I checked the adopted standard was diverging speeds of 220 km/h. However, these motors all work in parallel, so the difference in switching time is negligible compared to a more basic type for general use. And these points work by bending one rail a little bit out of the way, bending another rail into place, and for high speed, pushing the frog (image for clarity) to the other side. This all unlike maglev, where you need to invasively rearrange the whole track, the replacement of which is probably several metres away.
High speed rail has enough compatibilities with regular rail to make sense.
To append, some examples of high speed systems with multiple manufacturers.
🇪🇺🇬🇧: Eurostar has, for the Channel Tunnel, two types of train built by two manufacturers. The old type by Alstom, the new type by Siemens.
🇪🇸: Just AVE has three different builders. The S-101 by Alstom, the S-102 by Talgo and the S-103 by Siemens. The S-102 is also used by Avlo, while OuiGo used Euroduplex sets by Alstom, and Iryo uses ETR 1000 sets built by Hitachi Italy.
🇮🇹: The ETR 500 is built by a consortium of manufacturers, several of which have been absorbed by others. The ETR 1000 has been built by a chain of builders due to mergers n stuff. And the NTV units are built by Alstom.
🇯🇵: Pretty much every major manufacturer of rolling stock has built at least some Shinkansen units. You'll find trains by Hitachi, Mitsubishi, Kawasaki and others all over the network.
🇨🇳: The first generations of CHSR trains were all imported designs, derived from others, both Shinkansen and European types. This allowed them to kickstart a domestic rolling stock industry for later generations of train.
🇨🇭: Once we lower our standards of speed a touch, SBB uses two types, one by Alstom built in Italy, one by Stadler built domestically. And trains from neighbouring networks ride into particular areas, each of which has their own builders.