Because there are lots of situations where the gowns would be used, but the return to a suitable laundry facility becomes an issue.

In a surgery, they often use re-usable gowns - but then, they're in the room with a single patient for sometimes hours at a time.

But the non-surgical environment in a hospital, nurses, MD's and RT's are moving from patient to patient, umpteen times a day. Using a washable gown would generate a huge amount of laundry.

It makes things simpler then to have single-use-then-dispose.