Mostly to retain pilots. W/O's typically don't have collage degrees so are not as competitive for the major airlines and therefore more likely to stay in for 20 years. That's the idea anyway, but as with most their ideas it probably won't work.
The 100% promotion to Major is to encourage pilots to stay in for 20 years. Before you had to play the game, do the professional military schools, work staff jobs, etc. Now they've created a professional pilot track in all but name. But I'm sure there will still be requirements to do some staff jobs. Pilots at the squadron level are still working 50-60 hour weeks trying to do staff work plus fly.
To show how they're screwing it up (again) the last promotion board from Major to Lieutenant Colonel promoted 3% more officers in staff jobs than the ones who are flying at the squadron level and risking their lives in combat. Promotion boards just can't get over the "Officership" thing. Staff weenies can usually get higher endorsement levels on their Officer Efficency Reports since they're rubbing elbows with Generals, while the folks at the squadron level don't, unless they really stand out.