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A certain amount of species tend to work that way, an example of it: having less females but an astounding amount of males, resulting on females copulating with more than one mate, up to a big number (Charadriiformes).
It could be a result of their evolutionary genes, needing less of X to keep living, sometimes it could be because of environment changes, like humans making the ecosystem go nuts because of deforestation (of course, that's just an example), or because nature fucked up and it became that way.
In the end, is probably mostly due to being in disadvantage by having a 1:1 (so, adaptability/evolution). In fact, I would guess that they can keep living exactly because of that: Sometimes less is better for them.
