email hides embedded knowledge and frail processes (Jane asks Bob for A in an email and only Bob knows that B and C are implied when Jane asks. If Bob is out, the process to ask for A can be completely paralyzed.)
no actionable meta-data (no easy way to harvest project name, members, status, dates, etc. Good luck trying to find out who you need to invite to an integration-of-two-systems meeting.)
no easy way to bring new people up-to-date, save massive retyping.
no easy way to know current progress without sending out a mass request-for-status email
history tracking problems (all it takes is one or two people to not hit reply-all, for an email history to be completely untrustworthy from any one inbox)
Naturally, people should aspire to make replacement systems as easy or more easy to use than the ones they replace. But email being easy is never a good argument for leaving a process in it. Particularly not project-management tasks.