Y. pestis (plague) is kind of a fragile bug that poses significant challenges in developing a practical airborne dispersion weapon. In culture, it lives in kind of a polysaccharide slime micro environment that it creates for itself. In WWII the Japanese reportedly used vectors (infected fleas) to disseminate it.
B. anthracis (anthrax) on the other hand is a very durable bug. It’s natural life cycle involves sporulating and lying dormant for years, waiting for something to infect. The problem for the terrorist weaponeer though is that it is not communicable from person to person.
Also, releasing plague, if it did spread, might do more damage in the Muslim world, with their poor sanitary conditions, than it would in the West.