Because of the outfit (velvet pants + sweaters) preferred by its main proponents - industrial managers.
To understand that, a little background is necessary. Under the central planning, the organization of industries and plants included two "sectors" - administrative/financial and production. The captains of the production sector (typically folks with engineering degrees) considered themselves as those who were actually "doing the job," but not "running the show." The latter was supposed to be the domain of the administrative/financial sector cum Party structures (whose role in theory was similar to that of the Board of Trustees here i.e. oversight of the executive officers).
The spectacular 'rebellions' against the 'communist' rule (1956, 1968, 1970, 1980/81) were in fact demands for greater occupational autonomy put forth by the captains of the production sector - the velvet wearing engineers. They often were hiding behind the backs of the blue collar workers who provided the bodies at mass demos and fought the riot police, but the velvet pants were the ones who stood to gain from any liberalization of the central planning regime. The 'velvet pants' finally succeeded in 1989 - hence the name 'velvet revolution.' (cf. Michael Kennedy, _Professionals, Power and Solidarity in Poland_ (1992); Konrad & Szelenyi, _The Intellectuals and on the Road to Class Power_ (1979)).
wojtek