This is kind of two questions in one:
Why does there seem to be a trend towards longer passwords on sites today?
Why does Stack Exchange require passwords to be a certain length?
I'll tackle #1 first:
There are many reasons why a site (even one that some of it's users consider low value) would want to require password length. Length is the easiest way to require better passwords. Explaining to a user that they need to have at least one of numbers, uppercase letters, lowercase letters, symbols, etc is harder than simply saying "Your password must be 8 characters or longer".
Raising the bar on passwords might (not guaranteed) decrease fraud, contacts to CS, etc. There's also the reputation issue, well known sites that allow very weak passwords get dinged from time to time in articles and blogs. Adding on to this, a site may seem inconsequential today but 100M users later the perceived value can change. How many people knew they would care about their FB account five years later. Would you be comfortable if your FB password was "a"?
For #2 - Stack Exchange's password policy:
That's entirely up to them. One of two things probably happened, a lone developer picked a requirement off the top of his head and went with it or they had a discussion and made a reasoned decision based on .
Your choice as a user is to reject their decision and not use the site because they require 8 characters and you only wanted to use 6 or you can pick an 8 character password (and maybe let your browser remember your password).
One thing to keep in mind that not everyone shares your view of the world or your view the sites they use. Right now you have 1 reputation point in IT Security, when you have a few thousand you might assign a higher value to your SE account.