Countries that fund education more can pay enough to attract a larger pool of candidates, and then choose only to hire the best of that pool. Unlike the current state of most school districts in the US, who will hire anyone that can meet the minimum qualifications, but still have huge numbers of vacancies. Many districts have lowered the requirements so they can bring in even less qualified people.
From my reading, the Nordic countries view teaching as a high value profession on par with doctor and lawyer, and this results in potential teachers having to work very hard to land a job and keep it.
Canada (or at least Ontario where I live) pays teachers relatively well, hire on merit, but results are quite mediocre when measured objectively (and, for what it's worth, correlate with my own subjective anecdata). This is at least partly due to perverse incentives introduced by the seniority system. You can hire the best, but if you promote and give raises to the ones who put in the most time, the best won't necessarily stick around nearly as much as you want them to.
According to the main rule of taxation, you're going to get less of what you're taxing. I we can generalize it and say that you'll get more of what you're incentivizing.
From my reading, the Nordic countries view teaching as a high value profession on par with doctor and lawyer, and this results in potential teachers having to work very hard to land a job and keep it.