Just a part of 2nd ed D&D that was carried through in IWD. As I understand it, there are two rationales for prohibiting classes from multi or dual-classing. First, like paladins and specialist mages (although in IWD specialists can dual for some reason, or multi if gnomes), some classes require an extraordinary amount of dedication and focus, so there is no time to even explore other opportunities. Second, in the case of a bard, the character has made a career out of doing a bit of everything and may be incapable of dedicating himself/herself wholeheartedly to one thing.
Admittedly, the reasons aren't entirely convincing, which is why 3rd ed (like IWD2) allows unrestricted multiclassing - which sometimes isn't too realistic either.
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