Crawl budget is constrained in order to ensure that a website’s server is not overloaded with too many concurrent connections or too much demand for server resources which could adversely impact the experience of the site’s visitors.
Every IP (web host) has a maximum number of connections that it can handle. Many websites can be hosted on a shared server, so if a website shares a server or IP with several other websites it may have a lower crawl budget than a website hosted on a dedicated server.