The higher mortality rate of SARS and MERS - counterintuitively - helped to contain it, as well as the fact that it was only spread by people showing symptoms.
Not implying anything: But this tracks with my understanding of how USSR and USA wanted to select bacterial/virologic bioweapons [1]:
An agent that killed very quickly e.g. Ebola is actually not a very good bioweapon. You want something with a long incubation period that is infectious before people are symptomatic.
Things that incubate in short order + kill hosts quickly "burn the epidemic out" before it can spread.
Ideally it also can survive outside of a host for some period of time as well.
