[mrcarruthers]

Apple doesn't patch every security hole in older iOS versions. I don't know what the criteria is, but my guess is if it's a major security hole, or an easily backported one, they'll do it, but if it's super minor or not backportable, they won't.

[saagarjha]

If I had to guess, I think it’s when they see a report that it’s been actively exploited.