For me it’s both protecting the integrity of the sport in the eye of the beholder and encouraging a wider distribution of good horse.
For the first, the big trainers who swamp races like this might be as honest as the day is long, but the suspicion is always there that they have set out who of their number is to win with the others there as backup or to facilitate their chosen one. Even when a trainer runs just two horses, how often is it that the unfancied one comes in despite the trainer talking the other one up, or the winner down, beforehand? Maybe I’m too cynical, maybe not.
So far as the wider distribution is concerned it might be a little unfair on the big trainers, but a monopoly in any field rarely turns out to be a good thing.