Why is there no High-Resolution Infrared Radiation Sounder (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: 

http://www.esa.int/Our_Activities/Observing_the_Earth/The_Living_Planet_Programme/Meteorological_missions/MetOp/About_HIRS_4/(print)

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.