In addition to what Val said, the other reason is that it's way easier to migrate to a new version of web server than to a new version of EJB server. Our sense in creating the two exams was that
servlet apps would be using 1.4 soon, and that EJB apps wouldn't be using 1.4 for a long time. (Especially given the state that the big vendors are in, in terms of 1.4 for EJBs.)
Believe or not, we were trying to take a real world perspective!
-Bert