In all earnest, I suppose it's because general fitness means, "fewer lethal or gravely disabling disease alleles".
And having more daughters means, "more embryonic lethal crap going on in the genome" so only daughters survive to term, and reproduce.
It's technically possible for an XXY baby to survive inheriting an otherwise lethal sex-linked allele, but they are infertile, so there you go.
Crunkatog on ESO
Bart331 balance suggestion: aztec: remove civ
Voltiguer: Ender, Sioux in 1.04 will be a top civ, no matter how many layers of Sioux goggles you put on
schildpad on Elephants: ...their mansabdar unit sucks so hard it looks like a black hole
Crunkatog on Steam.