Why is the rhythm of medieval plainchant difficult to reconstruct?

The rhythm of medieval plainchant, also known as Gregorian chant, is difficult to reconstruct because its authentic rhythmic style cannot be definitively ascertained from historical records.1

Unlike later medieval music that utilized rhythmic modes or standardized notation to codify patterns, plainchant is characterized by a "free" rhythm that lacks measure or regular meter. Its rhythmic structure is inherently tied to the Latin language, relying on the natural accentuation of the text and the dynamic quality of word groupings rather than a fixed, rational framework.234

Because specific details regarding how these chants were performed have been lost, scholars can only speculate on their original execution. While some theories suggest that basic rhythmic units may have had equal durational values grouped in irregular successions of twos and threes, these remain unverified. Consequently, the lack of precise rhythmic notation in early manuscripts leaves the true performance practice of plainchant a subject of ongoing historical uncertainty.1