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Cove3, I am sorry to be rather frank, but this is pretty much entirely made up. No, I am sorry, I'll rephrase that: It's an observation you (think you) made. Which is fine, but I haven't heard or read a anything to this effect anywhere. What is your credible reference making this not an observation but a product of your reference-based research? While not totally unreasonable, it's not even correct that it's impossible to fly a rover with wheels stored folded during cruise and EDL, and then "stand it up" on the surface. Actually the MER rovers did stand up only after the panels of the descent stage where unfolded, on the platform they so created. There is a video that actually shows an animation of this process as it occured with the MER rovers[1].
Indeed, the main reason for departing from the airbag approach for MSL was the mass of the payload, roughly 5 times that of each of the MER rovers, which is simply more than airbags could handle[2]. It was said during one of the pre-EDL engineering briefings, that the MER mission was actually close to the limit of what the airbag system could handle[3].
With the airbags not being a viable method, the reason for the Sky Crane to be selected as the alternative was to have the powered descent stage stand off from the ground, thus not blowing up too much dust which could interfere with the rovers' instruments, and that the Sky Crane approach keeps the center of gravity of the powered descent stage as a whole rather low, which is easer to manage than a high center of gravity[4][5]. Both factors result in a soft landing, which is a first for a Mars landing mission[2].
The "platform on long legs" approach was unpractical for the reasons given in the article page as it is now, so that is correct and backed by sources.
The more plausible observation is, therefore, that actuated unfolding/unstowing of the wheels on the ground wasn't considered because it was not needed at all, as a result of the decision to use the Sky Crane approach. It was not one of the reasons to use the Sky Crane though. (I do not have one single reference that exactly states that, though, therefore it's more the quintessence of the statements made by mission engineers in the briefings leading up to EDL. These briefings are available in the video archives on the NASA UStream channel[3].)
Anyway, why an option was not chosen by engineering teams vs. alternatives that were, is not strictly encyclopedic content anyway, or at least much less than the facts which choices were made and then why. Anyone with a deep interest in that, will certainly find some further reading on the subject. Behind the scenes, I'm sure there was rooms full of engineers debating quite a bit about stuff we'll never know. The one killer reason contra the airbags given in briefings and mission overviews was clearly the mass issue. The reason pro the Sky Crane was, believe it or not, it's overall simplicity[6]. After all, it left the rover pretty much ready to operate as soon as it touched down.
