Why are fewer human genetic disorders recessive?

It is not necessarily true that fewer genetic disorders are recessive. However, there are many ways to inherit a genetic disorder.

In autosomal dominant disorders, only one copy of the gene is needed for the child to inherit the condition. Therefore, if a parent has one normal and one mutated dominant gene and the other parent is normal, the child has a 50% chance of inheriting the condition. An example of this is seen in Achondroplasia, a type of dwarfism where only one copy of the gene is required for an individual to inherit this condition.

In recessive disorders, the individual must have two copies of the gene to inherit the condition. If two carrier parents have offspring, there is a one-in four chance (25%) of producing an offspring with the recessive condition. An example is albinism, in which a person doesn't produce any pigment in their cells.

Sometimes, a disorder can be X-linked and is passed down along with the X chromosome. Males have one X and females have two. If an X- linked condition is present on the X, for males since they are hemizygous for the X chromosome, if they inherit one mutated gene on the X chromosome, they will develop the disorder, and if they inherit the normal copy of the gene on the X, they will be normal. Females can be normal (XX), or a carrier with one normal X and one X carrying the mutated gene. The only way a female develops an X- linked disorder is by inheriting two copies of the mutated gene, one on each of her X chromosomes. Colorblindness and hemophilia are X- linked disorders.

Genetic disorders can be transmitted on the mitochondrial DNA. The genes in the mitochondria are passed down from mother to offspring and if there is a gene defect in the mother's mitochondrial DNA in her egg cell, at conception, the children will inherit this gene from their mother.

Disorders may be inherit due to aneuploidy. Sometimes a person has a missing or extra copy of a chromosome due to non-disjunction during meiosis. An example of a condition with a missing chromosome is called Turner's syndrome. This is a female missing one X chromosome. Also, a person may inherit two copies of a chromosome pair from only one parent. This is known as imprinting. An example of this is Praader Willi syndrome which causes uncontrolled eating and obesity along with learning disabilities. It is can be caused by a person inheriting both copies of chromosome 15 from their mother. 

However, research has shown that most genetic disorders are caused by many genes along with the environmental conditions that the person is exposed to. Examples of these conditions that many people seem to develop including-- heart disease, high blood pressure, diabetes and many forms of cancer among others. This type of inheritance is known as multifactorial inheritance.

Therefore, to conclude, genes for recessive disorders may be in the population in individuals known as carriers. However, in order to develop these conditions, an individual must inherit two copies of the gene in order for the disease to be expressed in the phenotype of an individual. For this to occur, both parents would each need to be either carriers, or have the disease themselves. That may be a reason why these disorders don't occur that often. However, it seems that multifactorial inheritance accounts for a large proportion of human disorders.