Here's why:
* Notation limitations: The musical notation used in medieval manuscripts was primarily focused on pitch and rhythm. It lacked the specific markings we use today to indicate tempo (like metronome markings or tempo indications).
* Performance practice: Tempo was likely determined by the performer based on factors like the occasion, the style of the piece, and their own interpretation.
* Interpretation: Scholars today can make educated guesses about the tempo based on the meter of the music, the text, and historical comparisons, but there is no definitive evidence from the manuscripts themselves.
Therefore, while we have a good understanding of the pitch and rhythm of troubadour and trouvere melodies, we can only speculate about the intended tempo.