At least partly because it's harder and harder to immigrate to the US and Australia. I don't know if the score will keep increasing, but there's clearly a "cap". I mean, an outland applicant with a master's degree, 3+ years' experience, an age under 30 and nearly perfect IELTS has a score of 478, which is just a few points higher than the current cut-off score.
But I'm pretty sure the situation is only going to get worse for people without any Canadian background as the proportion of PNP immigrants is higher and higher.