longer ron,
I personally don't think using the excuse of the Program's maturity, as a metric to judge whether it should be turning up to a couple of UK air shows, is valid in the slightest. To use your line of reasoning, why hasn't a full Tornado display turned up to a UK air show over the last few years, given that it's 40 years old?!! Comes down to priorities.
As rh200 points out, it's because there are more pressing issues. The USMC are the first Service to IOC F-35 so this is a major milestone for the Program writ large, regardless of how late it is coming.
So, not this year sadly, but I'd put money on it being here for the shows next year.