Because of the maintenance and upkeep. Also if you slack off on anything the effects will come swiftly with everything going at such a high rate. Constant pruning or battling some type of algae because you missed a dose of ferts, or had to go on vacation.
Ah yes, but as I said, things come up. People have to go places unplanned or planned and don't have the means to take care of a high maintenance tank, and there isn't always someone close to them who knows how to care for a tank. Especially not a tank that needs constant pruning and regular fertilization.