ECS uses a drastically different approach creating a page as a set of Java objects. Turbine uses a central servlet dispatcher. Cocoon and XMLC use XML for content creation. Different styles work better under different conditions. JSP dictates style.
Style is a dangerous thing to standardize.
Programmers tend to fall into this category, as well as a significant portion of html designers. JSP syntax, as it stands now, IMHO, is a drag to learn, type, and look at. Discussions like this may help expose this as a shortcoming that could be addressed. If so, that leaves a sizeable niche in the server-side java community for a decent template tool with a simple, elegant syntax. Yet this holds great promise for the future.