Why is "Commemorative postage stamp" trending?

Wikipedia Overview

A commemorative stamp is a postage stamp, often issued on a significant date such as an anniversary, to honor or commemorate a place, event, person, or object. The subject of the commemorative stamp is usually spelled out in print, unlike definitive stamps which normally depict the subject along with the denomination and country name only. Many postal services issue several commemorative stamps each year, sometimes holding first day of issue ceremonies at locations connected with the subjects. Commemorative stamps can be used alongside ordinary stamps. Unlike definitive stamps that are often reprinted and sold over a prolonged period of time for general usage, commemorative stamps are usually printed in limited quantities and sold for a much shorter period of time, usually, until supplies run out.