So there is something odd going on with measles. I haven't dug into the data, but from a couple of data points I've seen, it seems that measles mortality was declining for a long time even while measles cases were not. That is, there was a decline in the case-fatality rate, without a decline in cases. The disease was still around but getting less deadly. Then the vaccine actually reduced the number of cases.
So what was reducing the case-fatality rate? I don't know, but it might have been nutrition. There's evidence at least that Vitamin A makes measles less severe/deadly.
Sanitation, antibiotics, oral rehydration therapy, machine ventilation, nutrition, and so on.
It's not the measles itself that was the cause of most fatalities, it was the pneumonia, diahrrea, and other opportunistic infections that come with it.
So what was reducing the case-fatality rate? I don't know, but it might have been nutrition. There's evidence at least that Vitamin A makes measles less severe/deadly.