Because if you go into the Luxury tax you have to pay a certain amount of money for that privilege.
You can go over the salary cap and still have the MLE available to use, but if you go into the Luxury Tax which is set at about $70 million then you have to pay for that privilege
Its $ for $ at the moment, but in a few years time it gets really harsh.
Teams that continue to go into the Luxury Tax will have to pay even more, It go's up to like $4.25 per dollar your over or something like that for repeating teams.
Like I said its harsh