There could be several reasons.
- The website may not be properly submitted to search engines.
- Sitemap should be formatted properly otherwise search engines may not be able to read it and index the webpages.
- If there are technical issues, a website cannot be crawled or indexed, like broken links, missing pages, or a lack of structure.
- If a website has low-quality content or is not relevant to the user's search query, then it may not be able to get indexed by the search engines.
- New websites take time to get indexed.
Should check the website's Search Console account to understand the crawl errors, if any, and fix them. Also, check the website's backlinks which could also be a factor for not getting indexed.
