There's no danger for an mri unless you've had major surgery involving metal inplants. There's no ionizing radiation so there should be no risks for the fetus.

Cost and scarcity are big limiting factors for mri though. Mri machines are massive permanent installations that cost in the millions. They are already busy with people who need diagnostics of specific conditions. Putting every expectant mother in one would lead to absurd wait times. Whereas doctors offices can have multiple little ultrasound machines on carts that are easy to quickly acquire imagining as part of routine visits.