This is precisely the question answered by the OP. The answer is, "because there is a whole spectrum of things you might mean by 'diversity', of which 'number of distinct species' is only one extremum".
-edit- by the way, since it may not be obvious to everyone, the reason why an ecologist might care bout evenness is because extremely rare species are often not very important to the wider community. From an ecological function perspective, there is very little difference between my above example of the 99%/1% community and a community that is 100% species A. So an community with two, equally populous species might have more functional diversity than a community with one very abundant species and several more, very rare species.