A vast difference in the cost of the cartridges you are interested in.
The M24 and M40 at the time of their development were closer to sporting rifles. So, no clearance is required to build a rifle to similar specs. Some of the first rifles pressed into service with the Marine Corps as sniper rifles in Vietnam were basically Winchester model 70 and Remington 700 match rifles that had been in use as " recreational " firearms used in matches. They weren't "hardened for combat use". So the optics, barrels, etc were upgraded later. Them the M40 was developed out of experience gained in the field.