JT is good radio filler cause his songs are so bland and inoffensive. Most people don't really enjoy them, but they are easily tolerated. A smooth beat, his soft one note vocals. That's also why Not A Bad Thing is getting an insane amount of airplay despite struggling to go platinum.
A voice like Mariah's has always been much more polarizing, so it's understandable that radio avoids her songs. They are potential channel changers.
