My only guess would be lack of demand, so lack of supply. But, a quick search and pic shows there are varieties, though if the question is asking art-wise why they're always white... most likely, and oddly enough, they simply haven't thought about it.
A very odd thing considering that we've seen Japanese artist are "fashion-statement inclined" via sweaters and swimsuits.
Looks like this one's got a typo in the Japanese text. (see post #2138589)
I always thought her speech didn't seem to make sense. It says "kuro no kon" here (黒の紺) - literally "Black's Navy Blue" - but the revised picture fixed the radical on the final Kanji so it says "kuro no himo" (黒の紐) - Black String - which makes more sense.
UuuuumAh-
You were surprised!
I did it~Black navy blue~
There is a typo in the JP text. It should say "black string".