They couldn’t even quarry the Washington Monument out of a single color of stone. It’s not that visible in pictures but if you go see it on a sunny day it’s hard to ignore that stupid line in the middle.

If you take too long building a cathedral the quarry might exhaust itself in the meantime. So even if you keep to the design it might not look right.

> The outside facing consists, due to the interrupted building process, of three different kinds of white marble.

For some cathedrals that visible mismatch in the materials might be a feature, not a bug.

At least that's the case for the co-cathedral in Zamora, Michoacán which had its construction interrupted for almost a century due to the Mexican Revolution, the Cristero War and its subsequent expropriation by the government. In this context, the mismatching facade remains as a testament of the building's history.

https://en.wikipedia.org/wiki/Diocesan_Sanctuary_of_Our_Lady...