Developing web-apps on the Java stack adds costs at every level. It is more expensive to run / configure / support the web-server stack, and developers costs more per hour which makes vendors more expensive. But most importantly it narrows your pool of 'tinkerers' considerably. It is much less likely for somebody w/o much time or budget to bother wrangling Java to get Sakai running on their own. These tinkerers are what makes up the meatiest part of the 'hobbiest' open-source community which is what drives projects like Moodle. By opting for Java the Sakai folks have effectively limited their universe of participants to salaried employees of Universities and vendors. This is why they had to raise almost $7M to get the project going.