On PoW chains you can "invest" x money (burned by paying for hardware resources) to gain control over the network (51% attack)

On Steem you could do similar attempt by taking control over enough SP to vote for witnesses. Currently you have to risk value of your "investment" for 13 weeks. With 5 days, attacker (well, you can't even call him that way) can chose good moment (favorable market conditions, etc.) to take control over Steem platform with relatively low price and risk.

Keeping higher (current) lockout time for SP to be eligible for witness voting would solve that issue.