In fresco painting, why do artists focus on sections of the composition that can be painted in one day?

Plaster dries quickly

Explanation:

The fresco painting for the first time develops by the Renaissance artists in Italy. Artists focus on one section of the subject in fresco painting that can be painted in one day because it dries quickly. Fresco painting is a  difficult artwork which needs pace and accuracy while painting. It is a method of painting done using wet lime paste on walls.  

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artists need to create a realistic interpretation so that people in general can relate to the artwork. for a common people, an artist's impression gets increased only if he can create something which would look realistic. an abstract is difficult for a common people to understand whereas if a person can draw a girl with simple pencil sketch and if that looks like a real figure then the latter will be praised more than the former.