Im guessing it's both because of homophones in general and the fact that yslow is a deliberate play on this - "why is this page slow?"

Everybody says why-combinator, no? Certainly everyone I've ever heard, including interviewers, YC founders, pg...

Of course I say why-combinator. He was talking about interpretation of names beginning with `Y`, rather than pronunciation.

It's read that way, but semantically, it refers to an actual thing in computer science, so it's not just a cute name.

wait you don't? As a non native english speaker I do parse it as "ypsilon combinator" but read it "why" anyway :)