The longer a certificate is valid for, the longer an attacker has to try compromise it somehow. Relying on certificate revocation instead of short-lived certificates is not an ideal solution. As systems have improved such that regular renewal can be easily done without human intervention (and if done properly, with no less, or in fact possibly more, assurance of security) it makes sense to take advantage of this by reducing certificate lifetimes. Like single-use credit cards, to give an analogy that a non-techie man-on-the-street can latch on to.