It is quite odd how some things that should be obvious are not so purely because tradition has deadened our sensibility. For example, it has long been standard practice for medical research on animals to use only male specimens since it was argued that the presence of female hormones would add another uncontrollable variable in the testing. This seemed reasonable enough that it was uncontroversial until people started finding that when the resulting medicines were used on women, unexpected things happened. The National Institutes of Health now demands that all testing be done on both males and females, something that seems glaringly obvious now.
Stephen Colbert has more.
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One other issue: trials on humans tended to be on males because many of the trial volunteers were prisoners.
I had to fight like crazy to get my postdoctoral PI let me look at female offspring. They had already gone through treatment (as fetuses) -- all we needed to do was let them live. And then he threw my data out when I left. I am sure he is still merrily prancing along with his all-male colony, thinking he is still right.