Why is CPM not as popular as CPC?

The Cost per impression (CPM) model or Pay Per View is often appropriate if the advertiser is looking more for generating brand awareness or visibility for something.  What is more important in such cases is that people see the ad, not necessarily click it.

In digital marketing, Cost Per Click (CPC) has generally been perceived to be more accountable. People value clicks more than impressions from a business point of view. A click to a website also allows you to analyze what actions the customer took on your site.

You can’t do that only with impressions.

CPC