The problem with a physical standard like that is that you can't (easily) ship it to labs all over the world so that they can calibrate to it. At least when you use fundamental constants, each lab can set up their own equipment to produce the correct measurement.
Also, anything physical will shed atoms, which will affect the mass.
Cumpson suspects that because the kilos living in national labs have been retrieved and handled more frequently than the international kilo, more carbon-containing contaminants have built up on them over time.
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The problem with a physical standard like that is that you can't (easily) ship it to labs all over the world so that they can calibrate to it. At least when you use fundamental constants, each lab can set up their own equipment to produce the correct measurement.
Also, anything physical will shed atoms, which will affect the mass.
Or, indeed, get coated in atmospheric gunk.
https://www.wired.com/2013/01/keeping-kilogram-constant/