I'm no expert on the topic, but I understand most athletic careers have a very limited timespan; you're expected to retire by the time you're what, 40? Younger? A "temporary suspension" in this case would therefore be a lot scarier than it sounds - if long enough, it could be the practical equivalent of a lifetime ban, and however much time is lost would really be multiple times more than it sounds like.

I lifetime ban, while harsh, would certainly be a stronger deterrent. And if someone is willing to cheat once, it seems more likely they'd cheat again if a new undetectable drug came along.