The weather satellites Metop-A and Metop-B carry copies of the High-Resolution Infrared Radiation Sounder (HIRS), an instrument with a heritage back to 1975. Considering that Metop-C is part of the same programme as Metop-A and Metop-B, a and operational satellites in the same programme normally carry the same payload, why is there no HIRS on Metop-C?
HIRS is an old NASA heritage design that flew on both European and US weather satellites as part of the EUMETSAT/NOAA Initial Joint Polar System. The Inrared Atmospheric Sounding Interferometer (IASI) will take over the duties of HIRS, since IASI is a more powerful instrument with European design for a European satellite. You can see this discussed here:
My perspective on this is that historically the US was the international leader in environmental satellites. The Europeans now have better designs and more committed plans to environmental satellites over the next couple decades. So, essentially, they don't need our out-dated hardware anymore.