Mishnah Berurah — משנה ברורה

Out of linen - For the rabbis ruled [this way] since then he would be required to make linen ציצית strings for it and there is a concern that he might also make תכלת [colored] strings (which must be from wool) for it and that would be a prohibition of a forbidden mixture when he wears it at night, for then is not the time of the fulfillment of the מצוה. And even though nowadays we don't have תכלת, we hold that nevertheless the ruling [regarding linen] is not nullified. And many rabbinic authorities permitted this, for they held that the main ruling was only that [people should] not affix תכלת strings [onto a linen טלית] in order that they don't come to [have] a forbidden mixture... but [the ruling was] not on linen strings. And also nowadays where we don't have תכלת, that ruling is not applicable at all. And because of this, the Rem"a wrote that if it's not possible [to do it] in another way, one may rely on them (ie. on those that permit it).