mercutio repeatedly calls out, "A plague on both your houses!" Why do you think Shakespeare uses the image of a plague in these lines?

Mercutio is cursing the Montagues and the Capulets, as their feud is responsible for his death.

Also to add, the plague had just been tormenting the whole of Western Europe and so it is also a half comical reference and reminder to the awful plague recently bestowed and how the family feud is affecting the city of Verona worse or similar to that as the effects of the plague. There is also a lot of public fighting between the two houses and a lot of innocent people are caught in the middle a lot of the times.