Top 5 AdSense CPC (Cost Per Click) would drop or fluctuate for the following reasons


Lack of keyword – When AdSense cannot find a matching keyword for ads, you should expect low AdSense CPC. Most bloggers think they can wake up one day and start blogging – There is a place for orientation. Except you’re blogging for the fun of it, to make money, you must understand the use and strategic locations of a keyword.
User search history – This is one major factor affecting low AdSense CPC. If the majority of your users are a gambler, expect more of gambling ads except you turn them off. What I meant is, users interest would influence the type of ads being displayed on your page in correlation to the matching keywords. You can get a feel for what kind of ads are showing up on your pages from Review Center
Traffic from low CPC country – If your traffic sources are from countries like Nigeria and India, expect a low AdSense CPC because most often, users from these countries do not convert.
You blocked too many Advertisers Ad – This, in turn, leads to low-pressure bidding. So many bloggers believe blocking low paying ads will increase CPC. This is true only if you run a News, Sport or Entertainment website where unrelated ads appear on your pages.
Clicks from users are not converting – When your users click an ad without converting, that is no action was carried out after the first click – like filling a form, Google is aware and takes note of the frequency. This may lead to Google Smart Pricing
https://theawardnews.com/google-adsense-seo/top-5-adsense-cpc-cost-per-click-would-drop-or-fluctuate-for-the-following-reasons/