Because the 90mm Recoiless is a better weapon. Of course we had the LAW and what ever replaced it.
If you are armed with an RPG then that is about all you can carry. With a LAW, everyone can carry one or two. If you want a dedicated weapon, you had the 90mm Recoiless.
When we went to the TOW and Dragon, we got rid of the 106mm and 90mm. This took away a cheap HE shell for the infrantry. We still could develop a laucher or missle that could be used with the 106mm or 90mm recoiless and give a cheap shell to shoot at cheap targets.
IIRC the recoiless rifle was pretty popular around the Korean war.
The problem with the LAW was that it was a one shot weapon. The RPG is relatively cheap , very effective and reusable.