It's in a player's best interest to declare for the NBA draft when their stock is at its peak. A one-and-done prospect feels they have nothing to gain from a sophomore year...except skeptics they didn't originally have.
For some, selling yourself on potential is the way to go—give scouts just a taste without showing them the whole package.
A guy like Jared Sullinger might have cost himself dollars by returning for a sophomore year, where he showed minimal improvement and exposed himself to scouts.
For these one-and-done prospects, leaving after their freshman year will maximize their draft value.