Probably the availability of people with 5+ years of programming experience. It was 1959! :)
5+ years seems to me to be a strange expectation. That would mean you've had either multiple failing jobs and might be an undesirable, or you've been somewhere for 5+ years. If you've been there for 5+ years, what's motivating you to leave? Where do people expect to find these vast pools of highly-skilled jobless people who have experience with <software stack X> in <field which employs a couple thousand people nationally> within <narrow time window>? That they never find any seems to underscore how irrational the 'requirement' is in the first place, but I see it everywhere.
2 years = SDE 2
5+ = SDE 3, regardless of language.