Gun launched missiles were originally a crutch for guns insufficiently accurate to fight at long ranges, which is especially true for low pressure or short barrel guns like those of Sheridan or BMP-3. They are a pain in the ass to load, they are expensive, and if you are going to use an ATGM, you can make a better one if not restricted by the needs of being fired out of a gun (most notably the caliber, most ATGMs not limited in this way are 150mm+ in diameter).
The biggest restriction being warhead size, as in shaped charges penetration is directly related to the diameter of the charge.
So long story short, why bother with a gun launched ATGM, when you can make a straight out better ATGM for the same price just to stick it on an IFV.