^
What he said.
+
Expensive mixers can have REALLY good preamps, so essentially youre getting a mixer with A LOT of flexibility (many channels, inserts, subgroups, aux sends, effects sends, etc, etc...) with all the preamps youll ever need included.
Plus their EQs, compressors, gates, limiters (if they have them) usually eliminates the need for a lot of outboard gear.
In the end I would say that it is basically because they give great sound, last long and well, if you wanna track 24+ channels you need 24+ inputs generally.