Over the past century, scientists batted around ideas explaining the scarcity of asexual reproduction, which led to numerous hypotheses regarding the costs of asexual reproduction. Here is a list containing many of these hypotheses.
“If we bear in mind that in sexual propagation twice as many individuals are required in order to produce any number of descendants and if we further remember the important morphological differentiations which must take place in order to render sexual propagation possible, we are led to the conviction that sexual propagation must confer immense benefits upon organic life. I believe that such beneficial results will be found in the fact that sexual propagation may be regarded as a source of individual variability, furnishing material for the operation of natural selection.” (Weismann, 1887 p. 609)
"The advantages of sexual reproduction with two different sexes are not limited to genetic heterogeneity of the progeny."