Why wouldn't they? Measles isn't fatal, so there's no concentration of immunity due to survival. Now if you're talking the Black Death, that's a totally different story.
Measles can most definitely be fatal, it just has a better mortality rate than the black death had/has. Also, luckily the measles is in a family of viruses that has a low mutation and recombination rate, contrary to what Steven Soderbergh might lead people to believe, so you keep immunity your whole life.
When has measles wiped out 1/3 of a continent? If the Black Death is less lethal now, it's because most of the people without at least partial immunity, died hundreds of years ago.
Did you misunderstand what I wrote? Measles has a good mortality rate, I think it is only around 0.3% (3 deaths in 1000 cases). The black death had a mortality rate of around 55% which is almost 3 death in 5 cases and some people say 75% which is over 7 deaths in 10 cases. See now what I am talking about. Also, I never said anything about it being less lethal now. I am not even sure exactly what it was, they say it was Yersinia pestis bacteria, but if you look at it from an epidemiology and sociology standpoint that seems unlikely. It seemed to spread person to person and people who lived through Yersinia pestis bacteria outbreaks did not think it was the same. I know they cultured that bacteria from victims and it is possible that it was a very virulent strain, but it is also possible that the victims also had a Yersinia pestis infection as well as something else, like a hemorrhagic fever virus.