Why is this the only one that's not double-sided?

Wouldn't there be some sort of rules problem with a sorcery having an effect for the rest of the game?

The Epic spells (Endless Swarm et al) didn't really have an issue. These days that sort of effect is tied to an emblem, but that's only been used on planeswalkers thus far.

In theory, exiling the spell helps with memory issues, since then you wouldn't have to hide the card in your graveyard. Epic didn't do that... but that might be because it was pretty damn obvious that that was going to be the only spell that mattered from now on...

It's certainly not a problem for the rules. There's a different issue on whether it's a problematic design due to memory issues.

There is some degree of precedent on "for the rest of the game" effects that aren't tied to emblems. See Stigma Lasher and Praetor's Counsel. So this is probably OK.

Also, you could now template this as "Players have hexproof for the rest of the game" :)

I think emblems are almost a red-herring: "for the rest of the game" effects can usually be written with or without them; but you still want the effects to be mythic enough you don't forget, and I think this is fine.

OTOH, it doesn't really need to be "for the rest of the game": the other effects with that are riders on something else, but this could be an enchantment with hexproof and have 95% of the same effect (or an enchantment, and have 90% of the same effect). I also note that I'd probably rather have "gain 10 life" than "double my life total" on a one-off effect: the second is a lot more fun, but the first is what you need when you really need it.