IMO the league has figured out that a short contract is more advantageous to a player joining the Avs because in most cases the player's offense improves. Then the contract can be quite a bit higher either with the Avs or with another team.
But regardless, like others have said, both sides need to agree on a contract. Mitts's camp pretty well had all the leverage in these negotiations considering the very important need for a 2C and that they had just traded Byram for him. It was also very important for the Avs to keep the AAV as low as possible and the other side knew that. Longer term would have pushed the AAV above 6M IMO and with the uncertainty of Nuke's and Landy's future with this team it was hard for them to go there.