Because a good player who feels he has an advantage over the other players should always buy in for the max. If you come to a game that is $100 max with $600, you have bought 6 buyins which normally would be sufficient to ride out any variance in the game. However if the limit rises to $300, you now have 2 buyins so variance will play a larger role.
In the above example, the big stack would have $600. So in that scenario a $100 limit is fine with you? And as the big stack, wouldn't it be better to see players buy back in for more than one-sixth of your stack?