Why should YC be involved in areas where it has no experience?
The money YC gives is minimal, so the main benefit for YC involvement is experience. If YC has nobody who has expert knowledge in biotech, it would be waste of everyones time to get involved.
Another thing is money. Most science startups need more than just people with PhD's willing to live with ramens. They need lab equipment, samples etc that cost tens and hundreds of thousands dollars.