Reason 1) If you receive a 4 year scholarship, you have one full year to decide if the Army is for you. As long as you walk away prior to the first day of your sophomore year, there's no obligation or payback required. So, for some percentage of scholarship winners, the Army forks over tuition for a year of schooling, then loses that future officer to attrition.
No such grace period exists for 3 year winners. The day you sign on the dotted line you are obligated to either serve or repay the scholarship. It lessens the Army's financial exposure.
Reason 2) when my son was a freshman, he was one of a handful of 4 year winners. 4 years later he was the only one to commission. The others fell by the wayside at various times and for various reason over the 4 years. At the end of year one, it was plain to see who was serious and who wasn't, and who had potential as an Army officer.
Year 1 of a 3-year scholarship winner's ROTC career serves as a trial period for both the cadet and the cadre. If a cadet realizes the Army isn't for him, he can walk away with no obligation. At the same time, it's an observation period for the cadre. Validating the scholarship at the beginning of year 2 is up to the PMS - they can and will decline to validate the scholarship if a cadet has not met expectations during that first year.
There may be other reasons, but these are probably the two most important.
On the battalion side, if a cadet does walk away, this leaves the 3 year scholarship locally, which can be re-awarded to those who truly chose "a different path." As mentioned above, the Cadre get a chance to try before they buy so to speak.
and common sense, its just plain cheaper for the Army. My battalion was mostly non-scholarship, as is most I assume. There were several in my commissioning class that started out not really knowing if the Army was the right fit for them, but ended up winning a scholarship and commission, a few were re-joining from other schools/battalions. Tons of dynamics, battalion size, zip code, social make-up, major, prior service, etc.