Why don't your happy customers write reviews? They (a) never think to do so; (b) forget to do so; or (c) don't know where or how to do so.
You can help! Create a "review funnel" process to ask, remind and guide your customers through writing a reviews on the sites you care about—and intercept unhappy customers before they bad-mouth your business online. Here are some examples:
See also how you might integrate the following features: