guard-and-reserve-officers-ncos-who-want-to-go-on-active-duty-must-apply-soon

I'm just going to toss it out there. The reason why junior enlisted probably are not included in the program is because there will always be an influx of new soldiers. Privates are literally a dime a dozen. However, there are too many slots being left open by NCOs and Officers that have chosen to leave active duty because they have just had their full of it, maybe their contracts are just ending and they want to move one, they are medboarded out for various reasons, or they are just fed up and don't want to reenlist or go indef. So, how does the army respond; offer to allow the reserve/guard guys and gals to walk on into those empty slots of course. But they don't need to let your SPC and below in on the action because the army machine in constantly pumping out new Joes fresh from training at a pretty constant rate.
That's why they are not included.

In addition to the other responses (with which I agree) junior enlisted are still learning their jobs, and fulfilling their Reserve or Guard contracts. If any/every Reserve or Guard junior enlisted Soldier were let out of their contracts early, there would be a huge gap in the Reserve and Guard components. Who is going to fill that gap?

There's also the fact that E1-E4 with less than 36 months of active federal service don't need this program. All you need to do is see a recruiter, see what PS options are open and start the paperwork to get a conditional release, at any given time we are tracking paperwork on 1 or 2 soldiers working on an release for active duty.

NCOs and Officers on the other hand rarely have been given the opportunity to transition to active duty and retain their grade and MOS.

Also, there is a lack of performance documentation available for Junior enlisted soldiers, for NCO's and Officers their OER/NCOERs, OES/NCOES 1059's, etc. allow for a far better overview of a soldier than the limited documentation available on junior soldiers.