Such is the nature of this fragile medium that most work in this medium from around the Renaissance has since been lost or damaged beyond repair. Many artists would also see such work as study pieces, not something to worry about preserving or presenting. Only some would see drawings as individual artworks in their own right rather than just a supporting piece to a larger fresco.
It is only down to the exceptional interest in the artist that remains today that any drawings remain at all. Indeed, all these years later we occassionally find newly discovered drawings being attributed to the label of Van Eyck and his studio. To be any more accurate in the present day would be exceptionally difficult.
Within the Northern Renaissance it was perhaps Albrecht Durer who was the first artist to understand the importance of drawing and would preserve them far more carefully than artists of previous generations. In fact he would even collect drawings by artists he admired, something never seen during the time of Jan van Eyck.
Within Van Eyck's early period drawings would be passed around studios as a means of instructing and inspiring others. Individual items of attention could be examined but the fragile pieces would slowly becoming damaged and ultimately discarded. Van Eyck is therefore typical in not having many left that can be attributed to him confidently, such were the times.