Why can some sacraments be valid outside the Catholic Church but not others?

Because the Church says so, and she can determine that the importance of salvation supersedes the ordinary organizational formula. In the order of salvation, baptism is the most important; the Eucharist, confirmation, and penance cannot save you until you have been baptized. Since baptism is the sacrament of salvation, and salvation is the most important thing on earth, the Church has to make provision so that people can have access to baptism even when they cannot get to a church or to Rome, or when they live where Catholic baptism is not promoted. She is not encouraging baptism outside the Church as the norm; normally we go to the church to be baptized, but when things are not normal, we may go outside her structures. And this is why anyone can baptize in an emergency, not because baptism needs no authority, but because the Church has authorized it; you do not need ordination to confer baptism, but you do need the Church's authority, and she has granted it for that situation.