Answer:
Mostly because of the material which they were made, but also because of their structure, their location, and the absence of fire contention (or the understanding of it).
Explanation:
Shops in the Middle Ages were traditionally located on the downtown, and the buildings were constructed using wood, straw, and bricks (but mainly using wood). The buildings were close to each other, and the streets were squeezed. The fire was common because of the lack of safety, the carelessness of several owners or employers, and mainly because of the building structure which allowed the fire to spread speedily.
