Because it has one serious limitation: <style> tags may not be nested inside any other custom tag. Many servers, including Suns reference implementation Apache Tomcat 3.2, are firing an exception like illegal flushing within custom tag. This stems from the way response- and writer-objects are handled in the JSP framework.
Other servers like Orion server tolerate db:style tags within other tags, but of course DbForms is obliged to the JSP specification and its reference implementation.
This is the reason why I am currently researching how to bypass the problem. Any hint from the reader would be highly appreciated!