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They wouldn't appear because magic in the snow is a must-have!
And the developers were just too lazy to create snow!
Enough already... Why is it a MUST-HAVE?????? Mages were in mountains and deserts when they first appeared in the second part (there wasn't even snow nearby). Then, in the third game, they were moved to snowy areas, and there were about as many complaints as there are now (although the complaints were more justified). In the fourth game, it seems that the screen depended on the type of terrain where the castle was located.
The developers weren't lazy; there have always been as many factions as there are types of terrain.