I don't think it's worth remembering why it was designed that way, in part because I think some of the original reasons for it were questionable at best.

But it does serve some useful functions.

One that comes to mind right away is it discourages exchanges from voting with custodial stake (although it clearly didn't discourage enough in the case of Steem, as exchanges were apparently persuaded that the rules would be changed afterwards to prevent them from suffering any consequences).

It also favors giving long term investors more say over governance decisions than it does speculators. On the whole, I think that is probably beneficial.

It's also saved a lot of people from losing all their funds when they've leaked their keys.