Attila

1. Time.
2. Technique
3. Imagination.

A formal upright needs a lot of the above. Any good bonsai needs them, but formal upright needs them even more.

1. As the previous posts suggested, it's hard to find good stock in advanced stage of development. If not specifically trained for this purpose, by the time a stock reaches the advanced age of six years (time for elementary school), the faults are overwhelming.
First, you have to grow your own stock, and then you have make it into a good bonsai. People don't have the patience to do all that.

2. Growing great potensai for this style requires specific techniques. It has nothing to do with styling, wiring, and the rest of the basic stuff. It is not taught in too many places, and although some bonsai books address the issue, it mostly develops with personal experience. So, most of the people have no clue how grow grow such material.

3. Formal upright doesn't have an interesting trunk shape. People find it easier to take a twig and twist it back and forth and as soon as they achieved an S shape they feel that they created a bonsai. Working with straight trunk and straight branches requires a more refined sense of shapes, forms, forced perspective and other tricks.

And finally, every bonsai must have dominant features which make the tree interesting. A formal upright must have a great nebari (good is not enough), great taper and outstanding branch structure.

Very few bonsai have all of these, while creating the impression of a straight trunk.
Personally, it's my favourite style. Maybe because it's such a challenge. But also because a good one looks very powerful.

Good thread,
Attila