Q. Why doesn't Travelocity allow a flexible search to international destinations, like Cancun for example?!

A. Travelocity doesn't allow international flexible searches because the US Department of Transportation objected to their practice of not including fuel surcharges and other fees in their fare results. As you know, these can amount to hundreds of dollars. So the DOT told them to stop providing international flex fares. You can use
zuji.com which is owned by Travelocity to find all kinds of international fares on a flexible date basis, over a range of 330 days from today, or
Orbitz.com for searches over any 30 day period.