Told that I need to get my abdomen xrayed now, why not ultrasound or mri?

What? Sorry but this question makes no sense. What were the reasons for the suggestions for these tests? With some info we could give you a reasonable answer. Sorry.
Specific indications. Simple inexpensive x-ray of abdomen still is a very important diagnostic stool, not out dated yet, like in intestinal perforations, obstructions, foreign bodies etc. Sonogram and MRI had their on roll. Your physician will examine you and determine which is best, safe, relatively inexpensive will order that test first.