Many people have wondered why, in 2012, the monarchy always went to the eldest male child. It hasn't really been a problem, since Elizabeth only had sisters and the next heirs were men no matter what happened, but now that the latest royal is pregnant we finally get an answer. The actual reason is that it's just a pain to change the rules.
In order to make it so Prince William's baby is his successor, whatever gender said baby happens to be, legislation has to be tabled in not only England but every Commonwealth nation. In Canada, each province may need to be on board, depending on how it is written in the constitution. It is, in effect, an administrative headache, one that is completely irrelevant unless since the next two heirs to the throne are both male. Barring a strange freak occurrence that kills off all royals barring the upcoming baby, these changes could be completely irrelevant for half a century. Nobody wants to touch it for the understandable reason that it's just going to be a pain for everyone involved.
Right now, and possibly nine months from now, the current laws are good enough, even though some might argue that they're patriarchal and backwards. It's still going to cost time and money to change them for no obvious purpose for a potentially long period of time. While a debate about whether Canada should continue with the monarchy itself is divisive, the idea that the heir should be chosen based on first born alone isn't likely to be contentious. We've had a Queen for an incredibly long time, and she's done whatever her job is well, so it's not like people need to get comfortable with the idea of a female head of state. The first male heir rule remains solely because of the stress of taking it out.
That problem explains a lot of minor problems people have throughout their life, not just the issue of monarchy. There are small inconveniences throughout our world that exist because the stress of changing the system is worse than the minor inconvenience of just dealing with the problem. It can apply to laws, product design, administrative structures, anything at all. It becomes less important that something works well, just that it works good enough for most, most of the time. Changing the system would require a widespread and expensive overhaul for a small gain, and many people are unwilling to take such a step because the perceived effort on their part is not quite worth the benefits. There are many examples of this in effect, from inconvenient placement of gas meters in some buildings to software where some functions are needlessly difficult to activate. The current systems don't work perfectly, but they work, and the time and effort it would take to change them is not seen as having enough of a benefit.
Eventually some of the results of this good enough effect get fixed, when enough people are annoyed and the change is seen as valuable. But it's perfectly understandable why people want to focus on larger problems rather than minor issues. When surrounded by things that are broken, it simply doesn't make sense to focus on the things that sort of work, and so we get those minor issues that pester us but don't really damage us. That is why it takes a big push to get those little problems solved.
