I've read that co-founders are the norm for YC. It seems to be a rare exception for solo founders to be accepted, you'd probably have to prove you're the next Google for that. But then even Google had co-founders.

I'd say don't take it too personally. If you search the YC companies directory it would not be easy to find a solo founder company. The lack of interest you saw could have been that they had already decided that you needed a co-founder early in the process.

It's actually quite good to have made it to the interview stage.