It is basically the same reason why our army are not using Bows, Flintlock Pistols or Hand Cannons anymore. They have better weapons. You will maybe still see some of them occasionally, but not as a staple.
And as heavy bolters are available to the Guard and are obviously easily mass produced, there is no need to issue Stubbers. You may find them on the black market or in some PDF armories (as you can find older and cheaper weapons in the hands of militias nowadays) but the Imperial guard as such does not have much use for it.
You may find some regiments who do still use them but as long as there is no more "doctrines" for Imperial guard, they go with the most average armory.
When the heavy stubber is the "to go" weapon for underfunded traitor guard and civillian cultist mobs, why should the Guard fall back to its use over the Heavy bolter?