I guess because the concert was on the 20th, but that explains neither Nash nor why the Rangers in the first place, unless the concert was in New York? (which I don't think it was?)
I've given it some thought. This is my conclusion.
"Girls, we're doing this concert on the 20th, so we need something with the number 20!"
"Let's use some sports team jerseys from the USA!"
*They Google, quickly realize that Rangers jerseys are fucking sexy, pick them/Kreider.*
"But wait, there are 7 of us. One needs to be special and front/center. We can't all be 20! Who's their best player?!"
*They Google, see that Nash led the team in points last year and as a bonus had exactly 69 of them*
"I'll be Nash. He had 69 points last year!"
"YAAAAAAAAY"
And then we got this video.