Why? In the year I spent instructing at Sumburgh, Shetland the minimum crosswind I sent students first solo in was 15kts(total xwind)!
Not only did they learn crosswind landings they all got to appreciate why trees don't grow on Shetland.
Well, my thinking is that someone trying to learn to land, will do so much quicker if they don't have to do crosswinds too. Once they learn to land, you can add the cross wind correction in then much easier.
I'm also figuring that when a student is sent solo to practice their landings that most instructors would want the conditions to be fairly benine.
If 15kts crosswind is normal conditions in Sumburgh then I suppose they might as well let cross wind landing at the same time as normal landings.
dp