What's the price difference between the Nozomi and the two slower versions? It's roughly an hour longer each step you go down, right?

My guess is that a lot of more price sensitive leisure travelers (vs business travelers) would take the slower train due to price if it's a significant cost to them (often because they're traveling in larger groups than a typical business traveler so more tickets to buy). And assuming they're taking the bullet train for the experience rather than look into the cheap inter-Japan plane tickets for tourists.