It's extremely low likelihood of death compared to other more lethal diseases. I'm pretty sure AIDS, leukemia or malaria are more lethal to children than measles is.
And as far as "the leading cause of death", diarrhoea or pneumonia are arguably the biggest cause of deaths in children.
Diarrhea and pneumonia are symptoms, not diseases, comparing them to a disease like measles (for which, incidentally, both are among the common complications) is not really sensible.
And as far as "the leading cause of death", diarrhoea or pneumonia are arguably the biggest cause of deaths in children.