Because they don't have a mold for it. A new mold for that cap would probably cost in the realm of $3,000+.
I was aware they didn't have the mould but it would seem like a strange omission, I was wondering if there was some reason it was less than ideal to produce (other than volume demanded).
Volume is the only issue, ISO is a minority. This is why most sets don't include all the ISO keys. There aren't enough users.
I wonder, if ISO with a full Euro keyset was offered in more drops, would ISO become more popular? A number of the group buys offer "ISO" which is some, but not all of the UK keys, and then none of the DE, FR, ES etc.
On PMK some drops offer a set for each country and some offer 1 giant Euro set. I'm not sure which is the best way to go. The giant set is more expensive but gives everyone a chance to get the keys they need.