My recollection is that the ISA is based on now well dated readings for North America.
The Industry has used a range of "standard" reference atmospheres over the years for different purposes, mainly test work.
As has been observed in earlier posts, we need two criteria to be met ..
(a) the reference atmosphere needs to bear some resemblance to what happens in the real world but is then idealised
(b) the driving requirement is to have a reference standard (whatever it may be) for purposes such as test reduction and comparison.
Much the same sort of philosophy as the aircraft design certification standards when you come to think about it ..