What is it?
A t-shirt.
Why?
To help charity through Watsi.
Why that design?
Because this is a shirt about hacker news and not ycombinator itself. Even though they are hosted in the same place, they are two different things and brands.
Also, notice that the only common design clue in hacker news is the orange top bar with the gray background. Hacker news lacks a logo. The nex best thing to use would be the only other thing that visually differences it: the ugly design.
Why not the ycombinator logo?
Because the shirt is about hacker news. Not ycombinator itself.