"dwindling support" From this article https://www.bbc.co.uk/news/uk-england-essex-45044933
There were rumours of closure hovering over it for many years as its location is a developers wet dream.
In its last year the owners announced that it was no longer financially viable and needed the support to keep it going. This was down to many complicated reasons and internal politics within the short oval racing community and, I presume, speedway as well as they didn't renew their lease.