Many gothic cathedrals still standing also are stronger than they have to be. Reason? Building was still guesswork at the time and the ones built too weak have long collapsed. https://en.wikipedia.org/wiki/Beauvais_Cathedral is an edge case.

So, it may be lack of structural engineering knowledge. They also may have intended to build something bigger, but realised they ran out of time or money, or the “something bigger” collapsed, and this is a rebuild, reusing parts.