After the death of Alfonso Carlos, the male line of the carlist line was extinct. According to the succession law followed by carlism, the isabeline line, ie, heirs of Infante Francisco de Paula come next. But carlism considered they were traitors to the legitimist line and though excluded from the succession. The next lines were the TS and the Parma lines, but some carlists did not want a foreigner as their King. So, they they chose Blanca as heir, as the closest female representative of their last King.
Thatís the explanation. It violated their law of succession, but carlism created a new concept, called legitimacy of exercise, which was complementary to the legitimacy of origin. Both were necessary to have rights.