The biggest gates for speedway were just after the War in the late 1940s. There were probably three main reasons for the decline in gates.
1. Entertainment Tax. Although sports were partially exempt from this tax, which at one time, in the early 1950s, went up to over 60% of gate money, speedway for some strange reason wasn't exempt as it was classed as a trial of speed and not a sport. 93,000 people witnessed the 1950 World Final with £22,600 being taken at the gate. If it had been a football match the total tax bill would have been £3100, because it was speedway the tax bill was £10,200. This was, of course, being repeated at all tracks up and down the country. The tax was crippling many smaller tracks. Either the promoters had to absorb it, which most couldn't, or they had to put admission prices up, which drove people away.
2. Television!
3. Just after the War people were looking for some excitement after the long dark days of the War. Speedway fitted the bill admirably, but, as time went, on I think the sheer excitement element of it palled for many people and just left the hard core supporters. Life was returning to normal and there were other things to do...