Why do sex-linked traits follow different patterns of inheritance than other traits?

They are located in the sex chromosomes (XX for females, XY for males).

Explanation:

In autosomes (the other 22 pair of chromosomes) we expect to have two alleles of the same gene, each located in one of the chromosomes of the pair.  These are called homologous pairs, so, the two chromosomes in the same pair will have the same information, in the same order.

The sex chromosomes are different. Even when XX are considered homologous, X and Y carry different information. So, for example, in the case of a recessive diseases linked to th X chromosome, the males will always display the disease if they have the mutation, since they don´t carry a correct copy of the gene, and the females can only be asymptomatic carriers, since even if they have a mutated copy of the gene, in the other X chromosome they can harbour a right copy or the gene.