Late to the party, but a lot of that has to do with what videotape cost back in the day. This was a problem that wasn't exclusive to wrestling, either, as networks would re-use tape for broadcasting, especially when it had stuff on that they didn't think people would re-watch. Best example of this is the first Super Bowl, as no full footage of the game exists (it got taped over by soap operas). It's very different today, as it's a lot more affordable for both companies and individuals to document just about anything. Stampede was mentioned by you and others and it wasn't just Stampede that felt that wrath here in Canada, as there have been some hockey games (famous ones in fact) from over the years that were never kept or had to be reattained in other ways/means.
This is why it always surprises me that Verne Gagne saved tape from as far back as he did (at least 1966, hopefully earlier). Must've cost him a fortune not re-using tape...unless the format he was using couldn't be used twice.
Even then, the foresight to try and save the footage is unusual in wrestling from that time frame, I think, at least in North America.
Now I have heard that his storage methods were sub-standard and a lot of what he had is no longer usable, which sucks. But again, at least he tried to hold onto it.