Why did the US Marine Corps use LAV while the Army adopted Stryker instead of sharing one chassis?
Short answerTheir priorities differed: amphibious and salt-water utility for the Marines, versus air and road mobility for the Army, with the programs also separated in time.
NotesThe Marine Corps needed a vehicle suited to maritime transport and operation around salt water, a requirement carried forward into its newer amphibious vehicles. The Army emphasized air deployability and road movement, and the two selections were also made in different periods, so a common chassis was less obvious than the vehicles' shared ancestry suggests.