Thanks for the questions and phrasing them respectfully. I appreciate that, AF.
Here are my reasons, and I will be the first to admit if I am wrong.
1. The QB market is high right now. If the Cardinals wait a year to explore trades for Leinart they may be risking three things: (a) the market won't be as high...thus the return they could get for him would be far lower; (b) what if Warner gets hurt and Leinart struggles replacing him? That would ruin any chances of getting good value for him or having any significant leverage in trade talks; (c) the $15M roster bonus becomes more of an imminent factor the following year...and could cause the Cardinals to release him without getting anything in return.
2. About the $15M roster bonus...and Joe makes a good point that whomever trades for Leinart may likely want to rip up his old contract and ink him to a new one...which, by the way, Leinart would probably be more than willing to do...but, if let's say the Jets offered their 1st round pick for Leinart...they could opt to try to negotiate a new contract with him--or--they could decide to keep his current contract as is with the realization that they were going to have to ink him to a signing bonus anyway (which they wouldn't have to pay until the following year)...also they would have one year to evaluate him and his value and be in a position to rework the contract after his first year there. In the meantime, they could save significant money for their cap this year only having to pay Leinart $3M this year and saving on the first round pick money. The Jets actually need the cap space after all the money they spent last year in free agency.
3. And this is speculative, I admit, and I know this is going to infuriate some of you, but I honestly believe that the Cardinal coaches think St. Pierre is a better fit for their system than Leinart. This isn't to say that the coaches are completely down on Leinart...I don't believe they are. But, it's more about style of play and Leinart's doesn't quite match the style they are looking for. If the Cardinals ran a more traditional offense, the way the Jets do, for example, Leinart would certainly be a better fit. But, the Cardinals have a high powered passing attack which requires the QB to be a quick reader of defenses and blitzes and have the quick release to get the ball out fast. St. Pierre has a quicker release than Leinart and he's got better feet...which is why he fits the system better.
4. And this may be the most important reason of all...with Warner back as QB...and the Cardinals needing difference makers on defense...if the Cardinals did have a chance to trade Leinart for a pick in the teens, they would want to do it. For example, if they were able to add Rey Maualaga or Brian Cushing...either of them could give the team a better chance of winning the Super Bowl, than a #2 QB on the sidelines would.
Honestly, AF, I have nothing personal against Leinart. I just don't believe he's the right fit for Whiz's system. In fact, I would bet you that had Whiz been the HC in 2006 he would have taken Jay Cutler over Leinart, and not have even thought twice about it.