The problem is, each variant differs in lots of different areas, not just the one you highlight. For instance, the S4 mini is almost certain to have a lot less power under the hood and a much lower PPI screen, which means someone who wants a smaller screen also has to compromise in a lot of other areas too. But this is Samsung’s intention as it drives customers to their higher priced, higher margin flagship.
Cost probably plays a big part in these decisions too; widescreen is now the mainstream standard so I’d expect it costs more to manufacturer 4:3 panels these days. And when you’re not Apple or Samsung (with huge margins that allow you to afford these luxuries), you don’t do it.
It’s a vicious circle. Customers can’t make a choice to use a different service because it doesn’t feature at the top of search results, and the different service doesn’t appear at the top of search results because customers don’t know it’s there.