Simply the word abridgement here leads me to wonder:

Was the continual abridgement of texts and excerpting small pieces for later use the partial cause of the loss of the arts of memory? Ars excerpendi ad infinitum? It's possible that this, with the growth of note taking practices, continual information overload, and other pressures for educational reform swamped the prior practices.

For evidence, take a look at William Engel's work following the arts of memory in England and Europe to see if we can track the slow demise by attrition of the descriptions and practices. What would such a study show? How might we assign values to the various pressures at play? Which was the most responsible?

Could it have also been the slow, inexorable death of many of these classical means of taking notes as well? How did we loose the practices of excerpting for creating new ideas? Where did the commonplace books go? Where did the zettelkasten disappear to?

One author, with a carefully honed practice and the extant context of their life writes some brief notes which get passed along to their students or which are put into a new book that misses a lot of their existing context with respect to the new readers. These readers then don't know about the attrition happening and slowly, but surely the knowledge goes missing amidst a wash of information overload. Over time the ideas and practices slowly erode and are replaced with newer techniques which may not have been well tested or stood the test of time. One day the world wakes up and the common practices are no longer of use.

This is potentially all the more likely because of the extremely basic ideas underpinning some of memory and note taking. They seem like such basic knowledge we're also prone to take them for granted and not teach them as thoroughly as we ought.

How does one juxtapose this with the idea of humanist scholars excerpting, copying, and using classical texts with a specific eye toward preventing the loss of these very classical texts?

Is this potentially the idea of having one's eye on a particular target and losing sight of other creeping effects?

It's also difficult to remember what it was like when we ourselves didn't know something and once that is lost, it can be harder and harder to teach newcomers.