Why don't people in Storybrooke turn back to themselves?

This was explicitly addressed in the show, at least in part, and the answer then was "we don't know". Emma specifically asks, in the Season 1 finale I think, why the curse was broken but most people hadn't gone back to their original forms, and no one can answer her. As far as I know, that was the last word on the subject -- we never get an answer, but it's potentially one of the things they're still waiting to reveal.

The general rule seems to be that people in Storybrooke are now an amalgam of their true form and "real-world" form. Physically they have all retained the shape and false life history that they were given by the curse, but they regained their memories of their true lives as well. Additionally, when Mr. Gold brought magic into the real world, the fairy-tale creatures were also restored to whatever fairy-tale habilidades they used to have. For example, the fairies are still humans but they have magic; the dwarves are still human but they can mine fairy dust; Ruby is still human but she can shapeshift.

Maleficent is a special case, as her curse-given form wasn't human to begin with. She, too, retained the shape that the curse gave her, though she may have gotten her magical abilities back and somehow Regina is keeping her prisoner anyway?