There already are/have been contagions that are as or more transmissible (in terms of R0) with much higher fatality rates, like SARS, MERS, polio and smallpox.

There already are/have been contagions that are as or more transmissible (in terms of R0) with much higher fatality rates, like SARS, MERS, polio and smallpox. But its difficult for a disease to get widespread or sustain itself when its both highly transmissible and deadly, because it often is instantly noticeable when individuals are sick, when outbreaks occur, and so its easier to contain and it burns itself out quickly.