Why would you? There are a tiny share of small screen size phones out there, and often you need a substantially different design for a small screen. Basically you are doubling the layout effort on your designers and app developers (or let's say 1.3 times modifier at least), and those designs have to be maintained as well.

The alternative is to really have a "one size fits all" design, but this will likely mean making compromises on larger screens.

Mobile development is expensive, and there's already enough to deal with to support backwards compatibility, and localization. It's just not economical to support small phones which almost nobody is using.