[kqr]

Another way to phrase the reply you have already had: because these assets are martingale.

In other words, to the extent good historic performance says anything about future performance, that effect is already priced in. This means you shouldn't hold on to a historically good investment just for that reason -- it's just as likely to be a loser going forward.

If you determined that the amount of risk you're willing to put into equity is 60 %, the only thing that happens if you let it drift up to 70 % is that you increase your exposure to equity higher than you originally intended.

Maybe you have good reason to do that, but historic good performance is not that reason.