For us reason was simple - latest Kayako version offers much more features that we were looking for. Kayako SupportSuite 3.0 gives us everything that we need in one place (i.e. tickets, task scheduling, teamwork, live chat and a whole bunch of other features). Also it makes it easier to reduce amount of tickets that we receive b/c it has instant search in KB articles and tries to match ticket text with one or more existing articles in knowledge base.
Cerberus is good and stable but lacks generic functionality which makes our work less efficient.