Heavy artillery units were armed with muskets because they were considered also to be part of the general defense of the fortification. And, of course, they didn't have to carry them around.
Field artillery just plain didn't have the room. The supply wagons didn't go "up front", and there was no room on the guns or limbers or ammo wagons for the 12 or so muskets the drivers and gun crew would have needed. The first picture shows a six-gun battery ready to move out. It was probably a late-war horse artillery battery, since the gun crew members are all mounted and the guns are 3" Ordnance Rifles. Not a musket or "artillery musket" in sight, just the sabers on the drivers, who rode the horses on the left side of the six-horse team. The drivers were not part of the gun crew; they held the horses in an engagement.
Everything was kept in its specific place and no space was wasted. Just an interesting aside - for a given gun, all the ammunition chests were packed the same, in a specific order, so that the number 6 or 7 could put his hands on the type of round called for with no fumbling even if he was new to the battery or was replacing a casualty. The Table of Fire for the gun was pasted inside the ammo chest cover.