There are multiple reasons why and MRI scan as a diagnostic tool can be considered inappropriate. The primary reason other diagnostics are used over MRI is because of cost-effectivity. The MRI scan is considerably more expensive compared to x-rays and CT scans. It would be inappropriate to ask for an MRI when the symptoms point to a simple bone fracture of the leg for example. An x-ray can give you the same amount of information in this regard without the huge cost of an MRI.
Another example would be if the patient is claustrophobic. Taking an MRI scan takes a long time and it would involve inserting the patient in a coffin-like machine. Getting an image would be impossible because the patient would need to hold still for a span of 40 minutes to 1 hour.
