38/44's are relatively uncommon because their popularity, and thus desirability and production numbers, waned quickly after the introduction of the .357 Magnum. As originally designed, the guns were meant to be used with a variant of the .38 Special round that, while producing much higher pressure and velocity, was dimensionally identical. While the ballistics were certainly there and the S&W N-Frame revolvers (as well as probably the .38 Special Colt New Service and Single Action Army) were more than capable of handling the pressures, many older and/or smaller .38 Special revolvers including the S&W M&P and Colt Police Positive Special were not yet. This prompted the development of the .357 Magnum and it's longer case to ensure that the new high-pressure ammo could not be fired in an older, smaller, weaker gun.
While S&W still cataloged the 38/44 revolvers (there were two the Heavy Duty/M20 and the Outdoorsman/M23) into the 1960's, the high-pressure ammo had long since been discontinued out of fear that it would be fired in an unsuitable revolver thus turning the 38/44's into nothing more than an exceptionally large and heavy .38 Special to anyone but a handloader. Because of this, the 38/44 wasn't particularly popular especially after newer lower-priced .357 Magnum revolvers like the S&W Highway Patrolman/M28 became available. Adding to the scarcity of 38/44's is the fact that many are no longer original as they were modified to fire other cartridges like .357 Magnum, .44 Special, or .45 Long Colt decades ago when they were relatively cheap and undesirable.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar