Yelp's goal as a business is to connect consumers with local businesses through trustworthy, unsolicited, and unbiased reviews. Soliciting reviews from customers (or even worse, positive reviews) violates what Yelp's terms of service and violates customer trust.

Since Yelp values quality over quantity, a few 5-star reviews are a lot better for your business than a lot of 3-, 4-, and 5-star reviews. You want people who are passionate about the great service they received to be reviewing your restaurant, not someone that is doing it for $5 off their meal.

Solicited reviews just aren't genuine. Not only can Yelp's algorithm can tell genuine reviews apart from solicited ones but potential customers can tell them apart too.