The eternal salvation of man is only in God. Man's rejection of God, if it becomes definitive, leads logically to God's rejection of man (cf. Mt 7:23; 10:33), to damnation.
Can the Mother who with all the force of the love that she fosters in the Holy Spirit desires everyone's salvation keep silence on what undermines the very bases of their salvation? No, she cannot.
And so, while the message of Our Lady of Fatima is a motherly one, it is also strong and decisive. It sounds severe. It sounds like John the Baptist speaking on the banks of the Jordan. It invites to repentance. It gives a warning. It calls to prayer. It recommends the Rosary.