There are a couple of reasons why:
1. Almost all the theme developers are being inspired by another custom theme (every Sublime Text user knows something about Soda Theme), but not the Default.
2. To my knowledge, syntax theme can affect only colors of the tabs and tabset (theme development is not well documented, only source code of the Default theme and some messages on the forum).
3. At first glance it's hard to understand how it works.
4. More styles, more layers to implement interesting tab appearance, sublime limitations (e.g. max number of layers is 4).
As I know only Minimal Theme fully supports this behavior, but it's not maintained anymore.
These themes have partial support of this behavior. Want to know why?
See (different color schemes per file type)
Boxy Yesterday Theme
✱ THE SCREENSHOTS CAN CONTAIN SOME GIF RECORDING ARTIFACTS (E.G. SIDEBAR BACKGROUND WITH SOLARIZED LIGHT COLOR SCHEME).