The alleles found in haploid organisms cannot be dominant or recessive. Why?

Because haploid organisms have only one set of chromosomes

Explanation:

Haploid organisms, unlike diploid that contain two sets of chromosomes (2n), have only one set of chromosomes (n). This means that they only have one gene copy and one allele (variant) for that gene. So, there is no recessive or dominant relationship.

Humans are diploid organisms, meaning that their cells have 2n chromosomes with the exception of gametes. Gametes are formed during the reduction division (meiosis) in which chromosome number is reduced to half.

So, when to gametes fuse, during the fertilization, formed new organism contain two sets of chromosomes (one from father and one from mother).