Why aren't sealed hub bearings common place?
Cone, ball and cup aka traditional wheel bearings are the simplest and cheapest expression of wheel bearings. Simple to manufacture, easy to maintain and repair with the simplest tools and a handful of standard parts that have been around for more than a century in the remotest locations. And the wheel will even turn after emergency repair when, say, one bearing ball is missing, while a toast sealed bearing means a break-down.
Sealed bearings require high precision manufacturing, logistics &amp; stock-keeping, special tools and trained mechanics.