A few things to consider:
Arizona's flexibility: The Cardinals have until March 17 to decide whether or not to pay a $7 million bonus to keep Kevin Kolb for 2012. The Colts have until March 8 to decide whether or not to pay a $28 million bonus to keep Manning. The time between those dates would let Arizona consider its options if the Colts decided against paying Manning.
Manning's health: Yes, Manning was cleared to resume his career. That is a positive step, but it's tough to know when or even if he'll be ready to perform at a high level. His neck injury is only part of the story. Nerve issues in Manning's throwing arm remain. Those issues could linger past the March 17 deadline for Kolb's bonus payment. The Cardinals presumably would not commit to Manning at Kolb's expense without feeling confident in Manning's ability to play well.
Moving up the timetable: Manning wants to know his fate before March 8. If any player has earned such consideration, it is Manning. The scouting combine is scheduled to begin in less than three weeks, right here in Indy. The combine would provide a perfect opportunity for team doctors from around the league to examine Manning if teams knew he would be available to them. The combine would also provide an opportunity for the Colts to embrace Stanford quarterback Andrew Luck, their likely choice with the No. 1 overall choice. If the Colts feel as though Manning will have a hard time recapturing past form, why hold onto him?
Quite a few people have asked about the potential for Manning landing with NFC West teams. Manning's health will be the No. 1 variable.