Why did Halo reach come out last

Halo Reach came out last because of the order in which things played out, Bungie got a contract to make the original 3 Halo's and were also allowed to make a few extra Halo's (ODST and Reach) because 343industries got the contract for halo 4, 5 and 6. ODST was a test to see if players could stand halo without being master chief and it worked so they proceeded to make Halo Reach.