It's partly because the game developers know that when you reach a certain level, your scrolls buildings will get bigger. So then you need more space, so you might spend diamonds to buy an expansion. Also, as they upgrade, they use more culture. You then need more space to add more culture items. So you might end up having to spend diamonds to add expansions and premium culture items. It's all designed to make you spend real money to buy virtual things. They have to make their money somehow, to keep the company going and pay their employees.
If you have the space, you can build a third scrolls manufactory, complete the quest with upgrades to 7 for each one, then later on, sell the third one to get some supplies and population back.
Or it may be possible to refuse the quest, so your game can move on to the next quest.