Top tier teams are usually the ones with the regular four figure fan base...
With some obvious outliers in tier two..
Hence they can afford to change nights and still, hopefully, keep enough people attending for the Speedway to remain at leasf affordable if not profitable.
Eastbourne were not one of those regular four figure attendance teams, hence the Dugard family would put tens of thousands in some seasons, and running at weekend would bring in around 700-800, tops, I would think, (after an initial novelty period that may bring a few more)...
Moving to a Monday or Thursday, from a weekend would then be a big risk, especially given the 30% and more increase in rider costs...