Because you have to replicate the same complicated mechanism for all the cars. Moreover, if a gyro in one of cars malfunctions the whole train is at risk.
Another problem is that the system is reactive, meaning that there is a lot of shaking and balancing that is quite unacceptable - think about drinking a cup of tea in such a wobbly train even on straight lines, while on curves it's not staying horizontal by design.
Yet another problem is that it can't share the current infrastructure without major switch changes while sharing the infrastructure the other way around is impossible.
And in the end all of this effort for trying to fix a problem that doesn't really exist, at least not in Europe where most of the rail infrastructure was already built 100 years ago.
Another problem is that the system is reactive, meaning that there is a lot of shaking and balancing that is quite unacceptable - think about drinking a cup of tea in such a wobbly train even on straight lines, while on curves it's not staying horizontal by design.
Yet another problem is that it can't share the current infrastructure without major switch changes while sharing the infrastructure the other way around is impossible.
And in the end all of this effort for trying to fix a problem that doesn't really exist, at least not in Europe where most of the rail infrastructure was already built 100 years ago.