The details of why and an entertaining read:
https://buildingscience.com/documents/insights/bsi-047-thick-as-brick
Note the warnings from the first article all through this article:
https://buildingscience.com/documen...on-masonry-walls-final-measure-guideline/view
2.2 Exterior Insulation of Mass Masonry Structures
Retrofitting existing buildings on the exterior (Figure 4 through Figure 7), is the best possible technical solution: exterior insulation provides the highest level of durability, energy efficiency, and comfort with the least technical risk. Specifically, externally-applied insulation and air/water control layers have the following advantages:
The insulation and air/water control layers can easily be made continuous and thus protect the existing structure (masonry) from rain, condensation, and temperature swings
Thermal bridging at floors and partitions is eliminated
Thermal mass benefits are enhanced
Access to conduct the work is often easier