Well, it was stated that one of the ways that Madoka is special is that her wish can actually break the laws of the universe. It could be that this is the first time that someone with this level of potential has been discovered, so it hadn't even been considered as a possibility, especially since Kyubei's people don't really understand magic anyway.
Also, Kyubei has those rules in place that prevent him from suggesting wishes, and I doubt he can change or alter those rules. It could be that the wish/magical girl thing doesn't work unless the girl makes the wish on her own. Magic can be finicky, sometimes.
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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber." --Krityan Elder, Tales of Vesperia.
Also, Kyubei has those rules in place that prevent him from suggesting wishes, and I doubt he can change or alter those rules. It could be that the wish/magical girl thing doesn't work unless the girl makes the wish on her own. Magic can be finicky, sometimes.
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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber." --Krityan Elder, Tales of Vesperia.