Any number of reasons, but the one which comes to mind is capacity. These days, many airfields are capacity constrained and simply cannot handle unplanned arrivals (unless emergency exists). eg. the international airport I work at (as per NOTAMs).
It could be as simple as assessing what services will be required. Is it a tech stop, a toilet stop, or an arrival?
Last edited by ferris; 12th Dec 2012 at 17:11.