Due to the shape of the okie it wiggles in the water. This wiggling is erratic and seems to trigger bites from steelhead. It's a combination of the visual, looking like eggs, and the erratic movement that makes them effective.
The nostalgic side of them is undoubtedly what has been driving the retail value of them in recent years.
I still use okies to fish with, but only the foam ones, or old beat up originals that I have repainted. Repainted okies can be very effective, but have little retail value.