If external bearing BBs are better, why use traditional ones? Related:

What are the benefits of external bottom brackets?
An outboard BB includes any BB which uses the same BSA or Italian threading as older style BB's, but places a larger bearing outside the frame, rather than a s,Allen bearing inside the frame. This increases BB stiffness and durability, while reducing weight. - zenbike

So if external bearing BBs are better, why don't all bikes use them? I have never seen a low-end bike with external BB. Are they inherently harder to manufacture? To install?
The average cyclist won't notice the difference in weight and torsion stiffness. Also most bicycles that are not used for sports purposes will not live long enough that their BB will ever wear out that much that it has to be replaced, therefore also low wear is not of huge importance.

So why are conventional BBs cheaper?

It's the question of a closed cartridge (conventional BB) vs. a (some kind of) open (external BB) system. One huge advantage of the closed cartridge approach is that the manufacturer has less points where they have to meet exact dimensions: for a conventional everything is OK as long as the threading is right and the axle is square – especially the axle can handle some tolerance at the square cones where the cranks are mounted. The rest of the internals can be constructed more or less freely which makes work easy for the manufacturer. 

In the external case the axle is part of the crankset, but not necessarily from the same manufacturer. Nevertheless it must fit very exactly into the bearings of the BB. That means the lead-though for the axle as well as the axle itself have to be worked very exactly to fit together also among different manufacturers. Above that, if the BB consists of two halves (Shimano Hollowtech II) then the two halves have to align very well towards each other. In sum there are more production steps that require higher precision compared to a conventional cartridge BB which makes stuff more expensive.

Another point is, that in the closed system approach the BB itself stays encapsulated if you remove the cranks. In the case of the external BB the BB is somewhat open after removing the cranks which requires to be more careful when doing maintenance work. That means it's either more time and effort for the mechanic that you have to pay or if you do it yourself there are more points where you should know what to better not touch or to keep clean.