Recently, I had to submit issues to some other projects that use other bug tracking system. I found that some of the other issue tracking systems are clumsy and cumbersome in comparison to mantis, and found it difficult to figure out why anybody would still be using these systems.

Of course, then it was obvious - History.

I believe most administrators of other systems would love to switch to a system as clean and easy as Mantis, but the limiting factor is having historical information in a legacy system with no easy way to migrate this history to Mantis.

For proprietary systems, I can picture those admins to be on their own, but for open systems such as Bugzilla or Track, where the schema is published, some effort could be done to create a migration script to convert the legacy information to the Mantis schema.