Google doesn’t show ads for keywords with very few people searching for them. This is because Google wants to give users helpful and relevant search results. If not many people search for a particular keyword, there may not be enough ads related to that keyword.
If there are not enough advertisers to have a “competitive” auction, then Google will choose to have no ads shown versus showing one or two ads.
The good news is that this presents a great SEO/Organic Search opportunity. Even though Google doesn’t show ads for low search volume keywords, you can still find regular search results like web pages or other related information. These keywords should be less competitive and easier to win top-of-SERP page results organically.