Most happy customers never leave a review because nobody asked, or the ask came from an unfamiliar address that went to spam, or it arrived with no link and the customer gave up at the search box. The businesses that climb in the map pack ask consistently, make it one tap, and follow up on every review that arrives.
Google also has rules here that catch people out. You cannot offer money or discounts for a review, you cannot ask only the customers you expect to be happy while steering the unhappy ones elsewhere, and you cannot post reviews on a customer’s behalf. A request that goes to every customer with the same link is both the rule and the thing that works.
The other half of the problem is what happens after the review is posted. A profile where the owner answers every review, including the awkward ones, reads as a business that is paying attention, and Google weighs reply activity when it scores the profile. Asking without answering leaves half the value on the table.