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In Interstellar, Cooper mentions that they don't have any MRI machines while talking to his kid's teacher. The dialogue goes as follows:

One of those useless machines they used to make was called an MRI. If we had any of them left the doctors might have been able to find the cyst in my wife's brain before she died, rather than afterwards. And then my kids could have been raised by two parents, instead of me and their pain-in-the-a#$ grandfather.

So, they had MRI machines in the past, but not any longer.

Why are there no MRI machines available? These are not like an Apollo mission which they have to declare fake. What happened to them? Is it ever explained in the movie or by Nolans or someone from the production team?

The second cause is that all of the 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.

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 extinguished or hobbled on our planet. It was a very depressing afternoon. [Laughs] I remember being struck by the fragility of life here. Everyone who has grown up in the West and has been fortunate enough to live through a rather peaceful period, every year everything seems a little better. It's hard for us to imagine periods when things go backwards, but they do very, very frequently. Just in the last 2,000 years, we can identify at last half a dozen periods in western culture where technologies were lost that ancient civilizations had that we still don't fully understand exactly, so you know that there's been knowledge lost since as early as the Middle Ages. What we know about that period survives because of beautifully transcribed manuscripts out on some rocky island on the North Sea. Although it's not our experience, it's frighteningly easy to imagine technology backsliding.

So it is basically an unexplained item that is in part thrown in there to indicate the downward spiral of the world.

This review explains this concept:

A script by Nolan and his brother Jonathan sets the stage, creating a near future world where crops are failing—wheat is gone, okra is on the chopping block, and all farmers can grow with any reliability is corn—and the planet is becoming a giant dustbowl.

They accomplish this visually with looming sand storms, as well as through small hints, mentioning food riots, hinting at hard times in the recent past, and painting a society that needs farmers more than engineers. In all of this there are a handful of nice touches, like how the New York Yankees are basically a high school baseball team, and though you’re never sure what happened, you get enough to know that the world has changed. Better now than it was, there’s no military, and things are relatively peaceful, but they blame for the disastrous near collapse of civilization on rampant technology, like MRI machines that could have saved Coop’s wife, or wasteful spending on things like space exploration—it’s now taught in schools that the moon landing was fake.