Probably because it was never listed as a powerful round from the factory and if you wanted something better than what was offered in the 44 Spl in the old days (245 gn round nose bullet at maybe 700'/sec, you had to reload it which most people didn't. And, if you are hyping something smaller, like a +P 38 load, you would want to be able to compare it to something bigger so it would sell.
The only thing you need do with a 44 spl is load it with a semi-wadcutter bullet - does not need to be +P, just a different bullet. Same thing with most 38 loads - but so many use the snubnosed revolver that the development of +P rounds in that caliber has been focused on their use in short barrels and many, including ammo makers, believe this a better defensive option for those guns.
If you load your 240 gn swcs to 800'/sec you have a very nice easy shooting and accurate load for your 44. If you want warmer loads, charges of Unique or 2400 powder will get that same slug moving to 1000 - 1150'/sec., in a good solid revolver. HTH. Mikey.