I had the pleasant of designing and implementing a portlet application back in 2005 - 2006 circa in Java with JSR164 (I may put the wrong JSR here). Initially, I thought portlet concept was very cool and could be a game changer. After a year or so working with it, my opinion changed: it was so convoluted, needlessly. If the goal is to shift the rendering responsibility to the portlet service owner then you could have used client side rendering with AJaX as well (remember AJAX?)
In the end, I think what really killed the portlet development is the complication it causes when you have to maintain states among the portlets.
In the end, I think what really killed the portlet development is the complication it causes when you have to maintain states among the portlets.