Deciding what instruments go on a mission is not done in a haphazard manner...
The resources available on the spacecraft (Size Weight and Power, SWaP, the spacecraft can support) are very limited. The negotiations and trades in deciding what gets on are very intense. I have set in on several such discussions for various missions and there are so many variables that all need to come together it is overwhelming. NASA certainly is not arbitrary in these decisions. Maybe I will write up something about how that sausage is made when I have the time...