I suspect it's because most people want a magnetic loop that covers down to 20 or 17m. A very wide tuning range with a magnetic loop is hard to achieve.
If you're lucky you might be able to make a 17m-6m magloop but I bet it would be more like 15m-6m, and it's only in the past few years that 15 and up have been any good this cycle.
So you can certainly build a 6m magloop if you're looking for a small 6m antenna.
The sort of 6m Halo antenna with added capacitance is like a magloop but often out of the of the "small loop" regime:http://www.nx7u.net/HiPar.html
If you want smaller loops, just scale the plates up.