Why not take them along? Is it because they refuse to wear Speedos?
Just the opposite. Because they do :eek:
These are the people who don’t have families willing or able to take them along. Some of them might not have immediate families at all anymore, for whatever reasons. (Last living child, dying childless and alone in a cheap apartment without AC.)
This is a large part of the answer.
But it must also be recognised that they were fully aware that heat or sunlight could cause deaths and did explain particular deaths in that way. It is just that they did so in language now unfamiliar to us.
Consider, for example, John Graunt’s tabulations of the causes of the death in the mid-seventeenth century bills of mortality for London. Which is the equivalent category for sunstroke?
[SPOILER]Answer: Calenture.
(Classically, this was a condition associated with sailors, but in this context it must mean onshore deaths from sunstroke or heat exhaustion.)[/SPOILER]
Other, slightly later terms would have included insolation, ictus solis and coup de Soleil.
What, not marthambles?
John Graunt’s tabulation is fascinating, but it would be a lot more so if we knew what some of these afflictions actually were. How would we go about finding out exactly what was meant by some of these terms? Is there a book of definitions someplace?
I think it is more newsworthy now than 50 years ago because back then it was kind of expected and in some cases unavoidable. Nowadays, if it happens, someone dropped the ball somewhere.
In much of Europe, very few apartments will have air conditioning. Why pay that much for something you only really use a few weeks of the year? Especially if those weeks you’ll probably be on vacation anyway?
As to why they don’t take the old folks along, really, how many of you regularly vacation with your parents? I’ve been on vacations with my parents and my in-laws as an adult, yes, but I don’t know anyone who considers that the norm unless they happen to be living with the older generation. So you go to the beach or whatever, figuring Granny will be just fine on her own for a couple weeks as she was last summer and the summer before that and the ten summers before that… and you maybe hear on the news, in passing, that your home city is experiencing record-breaking heat… but you don’t put two and two together and realize that somebody needs to get home and check on Granny until it’s too late. 
Er, the OED, or any other good historical dictionary. None of the terms used in the table was especially unusual at the time - the bills of mortality were written for a readership without medical training - and none is difficult to locate in standard reference works.
Of course, no one thinks that such statistics should be treated as infallible, but then, as others have already mentioned, statistics compiled from modern death certificates have their own complications. Any statistics on causes of death are necessarily imprecise.
But that makes no difference to my point. Which wasn’t that deaths from hot weather in mid-seventeenth century London can be quantified. Rather it was that there is little point in trying to explain why no one mentioned deaths caused by hot weather when, in reality, contemporaries clearly did attribute at least some deaths to heat/Sun-related conditions.