Heres an update right from Shane Turgeon.
They were released on the generic black cards simply
because at the time of their release, they were exclusive to Sears Canada and their regular production cardbacks hadn't been produced yet. Sears Canada and Kenner/Irwin had a great working relationship and because of that, they were granted an exclusive release on some of the new figures. There are 7 in total, 4 were released in 1980 (Tusk of Ugnaught, Han Cloud City Outfit, Dengar, and Lobot Lando's Aide) and they eventually came out on 41 backs.
In 1981 Sears had an exclusive on three more upcoming ESB figs and they were General Veers, Luke Hoth and R2 with Periscope. The "different" figure names was also a direct result of the fact that they hadn't been released anywhere else.
Veers is by far the most infamous but actually the most common. The rarest are Han, Lobot and Ugnaught.
Hope that helps...
Many Thanks for this info Shane.