I don't believe it's ever actually made clear why this is the case. After the most recent book I'm kind of hoping they get into it, because it's a pretty glaring and obviously intentional piece to be left out.
Bloodlines thinning out being the cause of reduction in the overall power levels is briefly mentioned (possibly speculated about) in the first trilogy I think. Vin's relative power level as a Mistborn was so much lower than *spoiler?* because his abilities came from the original source the same way the original Mistborn came into being, while hers were inherited generations later.
Edit: Weren't all the Noble houses descended from the original Mistborn? They were trying to keep the powers within the family, that's why having kids outside the family was a big deal? Dilution of the blood lines? Spreading of the powers outside the Noble houses?
What even are the spoiler rules here... those original 3 books have been out a while relative to the Wax and Wayne ones.
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