Mages have 2 advantages that only 2 other classes (and with legion only 1 other class) have, that is 3 DPS specs and being ranged. This virtually guarantees that at least one spec would be in the top 3.
Warlocks have generally also always had at least 1 spec that was great as well. Warlocks dominated most of MoP for example, and were really good in WoD as well.