My theory is that there are two factors...
(1) is that the Lightning will use a blown 5.4 DOHC variant and that both the Cobra and Lightning will get it appropriately tuned for each of course. I think SVE is dedicating its answer to the C6 Corvette with the new Cobra and they want it to make production with no problems.
Now (2) I think Lightning fans are going to get a major case of sticker shock when they see the production version. Perhaps Ford is putting it off to ease the pain when the roll it out with a MSRP close to the Ram SRT's sticker.
The bad thing about Ford waiting is that it gives Dodge time to reign supreme and get a healthy following of its own like the Lightning has enjoyed unopposed over the years. Plus, the PVO guys aren't going to sit on their hands because of the Viper. Chevy is going after it with the next Z06 so Dodge is rumored to be working on a Hemi V10 making upwards toward 600 hp and its a given that the Ram SRT will probably benefit from the Viper need to dominate its class.
I'm exciting about the future of domestic performance as long as gas prices don't continue to escalate. SVT vs. SRT is shaping up to be an exciting competition. Maybe one day, Chevy will wise up and join the party with SS models worthy to wear the badges.