Look at what hapened when odin or behemot were up, servers crashed etc. Also how would you handle a free roaming Leviathan ? It is like housing for everyone, it is not gonna happen (even if they were both highly advertised before 2.0 launch), their poor servrs can't handle all the stress. Probably will see them as a lv 3 LB for summoner.
Unique sumoning system, yes...