There can be several reasons, but some can be:

• Your site is new and hasn’t built up sufficient authority to rank for competitive search queries.
• Your content doesn’t match the searcher’s intent or lacks in-depth analysis or quality compared to the pages that rank for the queries you’re looking at.
• Your website is blocking search engines from crawling it (usually in your robots.txt file) or is instructing them not to index it (generally using noindex tags). Resolve these blocks, and you should see your site indexed.
• Your site has launched very recently (within a few days), and you have not submitted the site to Google or linked to it from anywhere, meaning it hasn’t yet been indexed. Set up Google Search Console and request indexing.