A weapon has to;
A. Be feasible.
B. Be the solution to a problem faced by the user.
C. Fulfill a role another weapon does not to equally well.
Otherwise, the weapon will not be developed or used to any great extent.
The OTL rifle grenade was developed to give infantry firepower beyond throwing distance (since enemy MGs had firepower beyond throwing distance) and the ability to take out lightly fortified positions (MG nests) without calling in own artillery (which would take too much time).
For disrupting enemy formations, artillery and skirmishers were used. In times when artillery operated within direct line of sight of the enemy troops, there's little reason to add this firepower to the infantry, especially as it may very well pose a threat to the infantryman's own formation should it be improperly handled, would be expensive and have little use against cavalry (no bayonet, cavalry moves too fast) and light infantry.