Why does each parent contribute only one allele to the offspring?

Because the gametes implemented in fertilisation are haploid in nature, in the case of humans, mature gametes contain 23 chromosomes instead of 46 which will then combine with another parents' 23 chromosomes during fertilisation, i. e: 23 (dad) + 23 (mom) = 46 (zygote)

Explanation:

A regular somatic cell has 46 chromosomes, 2n, 2 alleles but the gametes have 23 chromomes, n, 1 allele. The usual significance of this is so that there is diversity in the offspring and the offspring can gain characteristics (bad or good) from both parents.