One reason is because the rental places use white hose so it follows if you are wearing them your outfit must be rented. If you look at the Kenneth MacLeay prints for example there is not a pair of white hose to be seen, take a look through the Vintage Kilt Photos thread you will not see many if any white hose.

White hose have gained in popularity because they are cheap to produce, however with the myriad of colours available I simply do not know why anyone would settle for white hose, they seem to me to be rather stark and do not lend themselves very well to Highland Dress as it is rather colourful.