I think the issue with Cheltenham - much more than Crufts, or other events happening at that time - is how long it goes on for, and the sheer number of people involved in housing, feeding and providing drink for the crowds, not only in the racecourse, but also in the town, over a period of a week.
My impression is that the organisers naively looked at the risks of 'the time spent in the venue, and paid lip service to mitigting it in the form of hand sanitising stations etc, but gave no thought whatever to the risks outside the racecourse itself. Those working in bars, pubs, hotels, policing the streets etc, who will mainly have lived within Glopucestershire but perhaps not in the town itself, will then have spread the virus to surrounding areas of the county.
AFAIK, Crufts isn't surrounded by quite the same level of heavy partying outside the venue... but I may be wrong!