Lanefan
Victoria Rules
I suspect it's because a "class" like that is seen as trying to cover too many niches at once - it's a jack of all trades (and if not reined in properly, a master of all trades) that doesn't really need much of a party along to adventure with.I've always thought something like a "battlemage" should be a base class. I'm really surprised it's something that isn't added till much later in most editions.
The benefit of specialized classes (e.g. a Fighter fights but does not cast spells, a Wizard casts spells but really should stay well away from the front lines) is that they each have weaknesses that need to be covered off by others. Hence, the need to take others along into dangerous places; hence, the adventuring party.
Characters who can "do it all", either by inherent class design or by multiclassing, don't need the party nearly as much.
Lanefan