There are many poossible reasons, and it is not possible for us to give you a definitive answer.
Where your site appears in search results depends on various factors, including the exact search term used and the quality and number of your competitors. If you are in a high-competition niche, you'll find it harder to rank well.
I suggest you start with Google's "Search Engine Optimization Starter Guide" and ensure that you have done all the basics.
Also check your pages with the HTML https://validator.w3.org to ensure there are no coding errors which might make it difficult for search engines to crawl and understand your pages.