Our name was not intended to sound "cool", "hip", or "neat" in any way...in fact, attempting to do so would be self-defeative; "the zone" just isn't cool anymore.
Rather, our name comes from the 1967 Interstate Compact that formed the Washington Metropolitan Area Transit Authority. Title III, Article III, Section 3 of this document declares the jurisdictions that have become members of the Compact and thereby eligible for service from WMATA:
3. Washington Metropolitan Area Transit Zone.
There is hereby created the Washington Metropolitan Area Transit Zone which shall embrace the District of Columbia, the cities of Alexandria, Falls Church and Fairfax and the counties of Arlington, Fairfax, and Loudoun and political subdivisions of the Commonwealth of Virginia located within those counties, and the counties of Montgomery and Prince George's in the State of Maryland and political subdivisions of the State of Maryland located in said counties.
Before 1996, the "Transit Zone" did not include Loudoun County; the addition was made in anticipation of the Dulles Metrorail extension that will bring rail transit to the once remote Northern Virginia suburb.