Probably because it is easier to maintain support for just 1 database service, and to this I absolutely agree. Although with a good abstraction layer it would be easy as cake to use the RDBMS you want.
edit: Jo3's answer right below is much more precise
GOD Bon, maybe you'd like to check Open Tibia as they still maintain support for SQLite and PostgreSQL
you just would not have the bleeding edge features of TFS.