Round chutes (cupolas) are designed to get soldiers from the plane to the ground quickly.
Quickly enough to remain the least possible amount of time in the air where they are an easy target for any ground troop. (This is why soldiers are dropped from very low altitudes; 400m is usual, but some combat drops occurred at even lower altitudes)
But not too quickly that too many of the dropped soldiers end up unable to fight from the hardness of the landing. Please note that it is assumed that some will get hurt on the landing, and the calculus is designed to balance the risk in the air with the risk of the landing.
In contrast, rectangular chutes (wings) are designed to be dropped from higher than 900m, steered in the air, and to provide a very comfortable landing (as long as the surface of the wing is adequate for the suspended weight). They were also introduced for skydiving as a sport, and only later found some military use.
