Network effect. The important metric isn't how many reviews you have, but how many your competitors and neighbors have. If no one uses yelp locally, nobody wastes time on it. My daughter's optometrist has no reviews and the most popular optometrist in the county has five. Clearly a waste of time. My favorite "old time" hardware store has one review. Yelp seems to be for coffee and food. Observation shows possibly as many as 0.1% of coffee drinkers will write a review, and a local family run very popular burger joint that all the non-vegetarian locals have probably eaten at has over 0.03% of their visits reviewed. That is far higher than doctor rates or hardware store review rates. I drove past that restaurant yesterday and they had more local license plate cars in their parking lot than total lifetime yelp reviews... Clearly whatever it is they're spending their time on is more financially rewarding than boosting their yelp score, LOL.