For a lot of collectors, nostalgia plays heavily in their collecting habits. Having a sense of continuity in what they purchase is very important to them and for collectors of that bent, O-Pee-Chee (OPC) is one of their go to sets.
OPC has been producing cards in Canada and the United States in one form or another since the 1930s. Most collectors of a certain age (middle) remember the OPC sets of their youth which was a mirrored version of the Topps set.
The two companies would produce similar sets over the years with Topps being released in the U.