Earlier quoted context omitted.
Because the 128 a sweet spot and most people don't have a need for more. The 64 size is just enough to push most intensive users over the hump and in to iCloud subscriptions. I'm sure Apple has all the analytics on how much space is being used by their users and how much iCloud subscriptions would take a hit with an 128 offering.
Except I just noticed the new 11 is available in all three sizes (64/128/256). Whatever, this doesn't bother me so much about their phones. The non-upgradable storage in their laptops is a much bigger issue.
iPhone 11 (non pro) is available in 64/128/256GB capacities.
https://www.apple.com/iphone-11-pro/specs/ https://www.apple.com/iphone-11/specs/