Because it severely impacts all the other guests who don't shell out extra money for melted chocolate.
Does it though? Does it really? August and September crowds are already the lowest of the year. Having a night or two each week where you have to go to another park at night isn't a big ask. If you're only going down one day to experience Magic Kingdom, just go another day.
Or, hear me out ~ have fewer parties. *GASP*
They all sell out. I doubt this is even in the mind of anyone making decisions.