Diallo might have been a top 5 pick out of high school. Making him go to college for a year allowed the NBA to realize he's probably not a top 5 pick. I would say that's exactly what the intent of the one year rule is. Making players go to college for a year is not about developing them, it gives the NBA a year to scout them against college players instead of high school players.
Which makes good sense from the NBA's point of view. Scouting HS players has to be tough especially when they are playing against other HS players. Not to mention the resources involved in doing it. Some of these great HS players are exposed in a college setting.