This was due to the itinerant nature of medieval kingship and the similar conditions that prevailed in lesser administrations under dukes or counts.
There was rarely a single capital for a kingdom, and the monarch and his (or her) court would travel periodically between several seats of power during the year, taking precious goods and much of their furniture with them.
Furniture was constantly shifted around, stored and often disassembled to suit the role the great room was playing at a particular time in the day or the month.
These two forms are usually not employed as portable desks any more, but they are bought and sold as antiques or as reproductions and usually valued as much for their monetary worth or their aesthetic appeal as for their practical use.
At the other end of the scale, the 17th century saw the appearance of several other kinds of "chest" desks, such as those destined for use in ships or for getting paperwork done during a military campaign.