A tank wouldn't tow an artillery piece. Without ammo and crew, that gun is a useless trailer. A tank uses every nook and cranny to carry ammo for its own main gun, no room or racks to carry ammo of a different caliber.
105mm Shermans belonged to armor battalions. They provided direct support aritllery fire to the advancing tanks. The reason why they use the 105mm Sherman is so it looks like the regular Shermans in formation. A self-propelled howitzer is a high value target so it is "hidden" in a Sherman.
The 75mm armed M8 HMC based on the M5A1 Stuart light tank were assigned to light tank battalions and reconnaissance squadrons. Like the 105mm Sherman, its role was to provide direct support artillery fire to the light tank units.
The 105mm Sherman didn't replace the M8, they both had the same role, just in different type of units.