I'm fairly sure it has been to Cornwall in the past though. But I don't think it has been to Kent since it was the Milk Race and they certainly can't use the infrastructure excuse for that.
Yorkshire misses out this year too, as does East Anglia. It happens. It's a race of eight stages. Even with 21 stages the Tour de France usually misses out large parts of the country - Alsace this year, for example.
I would love it if the ToB did visit Scotland - the scenery would be a fantastic advert for cycling in Britain - but it would be a logistical nightmare so I get why they don't. It would be nice if Scotland had its own stage race along the lines of the TdY.
ETA: just checked and the last time it visited Kent was 2006, when a stage finished in Canterbury, the year before the Tour de France did the same. For some reason, I don't remember the ToB coming down here, even though I live just a few miles away from the finish line. It has visited Scotland many times over the years, the last time in 2015, but no further north than Glasgow/Edinburgh by the look of it. It seems I was mistaken in thinking it had visited Cornwall in the past - Exmouth is about as close as it has come.