Short Answer: They ran out of Isotopes
Long Answer: The way an MRI works is that compounds labeled with stable isotopes are used in Magnetic Resonance Imaging to render visible metabolic changes in the body.
The Isotopes are a scarce resource; it's already being discussed that the global medical community could run out of the Isotopes needed in this ...
Since good canonical answers have already been posted, here's my take:
MRI machines need liquid helium to super-cool their electromagnets into superconductors. Helium is a non-renewable natural resource and, in fact, we are already in short supply of it.
Presumably we ran out of helium, and without a suitable replacement, we scrapped the now useless MRI ...
According to Interstellar's screenwriter Jonathan Nolan:
Revelation 2: The death of tech in the film, like GPS and MRI machines, is based on informational extinctions in history.
Jonah Nolan: Kip and I spent a memorable afternoon with some fantastic scientists that Kip pulled together to talk through all the different ways human life could be ...
The cause is that all of humanity is concerned on making food to survive, so most of people are farmers, there is a shortage of engineers who could make and operate such machines, as well as the fact that it is very expensive to use MRI.
There is no need for MRI machines when most of the people are dying from starvation, not diseases which need MRI scans.