Why do you think deleterious dominant alleles are not very common?

A dominant allele can be described as the one which suppresses the effect of a recessive allele.

A deleterious allele is an allele which causes a decrease in fitness for a species as compared to the other allele form for the gene. A deleterious allele might be present often in a species but it might get masked due to the effect of the other allele. Due to this, a deleterious allele is mostly a recessive one. However, there can be some deleterious alleles which are dominant.