It would not really be a point to having Steem as Steem Power if you could just power it down immediately, would it? 

By having a withdrawn period, the price of Steem should theoretically be more stable, and whales can not just dump all their coins on an exchange at the same time. This allows the rest of the community to react to big sell-offs. 

Well, according to coinmarketcap right now at this moment steem has currently a total circulation of 267 million steem. Now more than 80% of those steems are currently owned by whales who are less than 5% holders of steemits population. 

Now imagine a scenario where for a conflict or some other reason some of the whales decided to leave the platform with all their non liquid asstes and which in this case are STEEM POWERS. If only they combindly take the total 10% of sp out of the circulation steems value to USD and Bitcoin will colapse to a level where it would be non profitable to use steem. 

So to solve from any "Holdable" situation power down is a failsafe Protocol. Hope this solves your query.