This has been debated on this forum before but I don't have the patience to search for it myself - have a go and you might find the answer.
I seem to remember, though, that it was more a structural consideration than anything else. It's also worth bearing in mind that the shape of the environmental envelope varies from ac to ac up there and it's always worth a check when the ISA gets down to -8C or so in the upper levels.
Over to those more clever than me!
Cheers all,
mcdhu