On any other site in the Stack Exchange network, if I try to sign up it offers to use my existing Stack Overflow login. I've used this feature many times; the upside is that you get an immediate 100 points on the site you joined and can start voting or leaving comments right away. Ask Ubuntu doesn't seem to offer this.
That's because you already have associated account there, and you got the 100 points bonus almost 11 years ago:
Since you can't associate the same account twice on same site, you also can't get the bonus twice, that won't be fair.