Irene
Having looked at a lot of businesses we have found a number of reasons:
1- Some industries are not very reviewable. Who wants to publicly proclaim that they just got dentures or a teeth cleaning? These industries fall on a continuum from dentist to restaurant. And a lot in between.
2- The business just didn’t stand out. Maybe the customer was happy enough to come back but not so happy that they are willing to risk their reputation on your behalf.
3- Similar to the above is the business that doesn’t have any or very many reviews. Few people want to step out from the herd, express their opinion and later be proven wrong.
4- Often there is a weak social link between the business and the customer. Just an email might not be enough to induce the customer into creating a review. Maybe there needs to be a personal ask as well.
5- As you can see it varies a lot by age group. It also varies by economic class and other issues. It could be that any given business’s core demographic or age group is not predisposed to leaving a review.
6- Finally, it could be that the review site, particularly Yelp, didn’t like the fact that they were an infrequent reviewer and decided not to show it.

From where I sit, the answer is straight forward, if not easy.
1- Run a great business that exceeds the expectations of the client. The shock and awe of exceeding their expectations goes a great length in helping get more reviews.
2- Ask every customer with a personal ask and follow up with an email or a text.
3- Keep asking and don’t fret over one or two or three that didn’t leave a review. Their reasons are, as noted, many and you just need to keep on moving on and asking.