In both greyhounds and speedway the problem in London was costs and the massive rise in land value. Stadiums just couldn't draw the crowds to make either sport viable anymore. Many sites were sold to developers.
Wimbledon was everything that scuppered both sports in a nutshell. Speedway went first as with many of the tracks the overheads were unsustainable. The GRA took over the lease of the stadium and the Greyhound Derby moved there but they allowed the stadium to just run down into disrepair. The last time I visited there I was shocked at how awful the place was, back in the 70s it was one of the best in London but the end came when the GRA gave up on it. The new Wimbledon football stadium was built on that site along with a lot of new apartments.
TV also did for both in that top televised events deterred people from actually going to them. If you watch the World Championship Speedway Grand Prix the filming and race format is a different world from the old days and some of the racing is truly spectacular, all at the push of a button...