In addition to crawl health and crawl rate limits specified by the webmaster, crawl rate will vary from page to page based on the demand for a specific page.
The demand from users for previously indexed pages impacts how often a search engine crawls those pages. Pages that are more popular will likely be crawled more often than pages that are rarely visited, or those that are not updated or hold little value. New or important pages are normally prioritized over old pages which do not change often.