Generally, you (meaning women, female dominated site, fair to assume) end up splitting up with someone who you've loved enough and thought special enough and good enough to have children with because either:
- He was making life more difficult by being a crappy father/selfish git in the first place and not pulling his weight.
- There was some kind of abuse ranging from being overly critical and/or a bit controlling to full on physical beatings.
- He had an affair.
- He has some kind of addiction e.g. alcohol, gambling, drugs, which are putting a lot of strain on the family.
In 1, if he didn't pull his weight before he's not likely to suddenly start when the relationship is over. So unlikely to take residency of children. In 2, you probably don't want the guy to raise the children anyway because abusive blokes generally aren't good fathers.
- is probably irrelevant but I suppose you could argue that he has already shown that he can quite happily compartmentalise the family so it's probable that he would prefer to compartmentalise the relationship with his DC and see them at weekends. 4 is fairly self evident that they would not be best placed to look after the DC (at least at the time of the split assuming that the nature/severity of the addiction is what caused the split)
The four can of course be reversed (although I think the selfish git/not pulling weight is heavily weighted towards men just because of society in general) and relationships can split up because of mutual agreement that both parties have grown apart, or that you weren't really suited anyway. But I think that the majority of women who split up from the father of their children would have done so for one of the four reasons above.
THEN consider that most couples do tend to set up in the SAHM/working dad thing, either literally as that, or with the woman working part time, or her job being more "disposable" than the man's, with her taking days off when the children are ill or to see Christmas plays or with her hours working around drop off and pick up times whereas the man's hours don't take that into account. The general assumption is that this will be the case, as well, so it takes a particular conscious thought/planning/discussion to go the opposite way or have a set up close to 50/50. Shared maternity/paternity leave has recently been introduced, but I doubt that many men have taken it up yet, because the expectation is still that a woman will go on maternity leave, not her partner.
SO, you have couples splitting up because of one of the four reasons I mentioned at the top. They're going to have residency awarded to the woman, mostly. Then you have couples splitting up because of mutual reasons, the woman's "fault" or some kind of combination. But these couples are still likely to have had skewed childcare responsibilities in the woman's direction, just because it's expected in our society. I bet it's a very tiny, perhaps even statistically insignificant number of couples who, before the maternity leave legal changes, sat down and discussed whether the man should give up his job after the woman's maternity leave ended and become a stay-at-home dad. Most people discuss when and whether the woman in a couple will go back to work - nobody asks about the dad, it's just assumed of course he will stay at work. So there's still a massive skew there, and it DOES make sense for the parent who's been doing most of the day to day care to keep residency.
I think the proportion of couples who decided, consciously or unconsciously, to have childcare be more equal or skewed in the man's direction AND happened to split up are probably pretty low. And that's why it looks as though women "always" get residency of the children.