Well, let's look at the science:
At the teacher level, student performance in math is predicted very well by the teachers competence in math. This has been confirmed time and time again. (Campbell et al (2014)
Studies found a big impact of the teacher competence in math on the competence of their math related pedagogy (Depaepe,2013).

This is of course no surprise: if you don't understand math, you can't teach math. Well, then let's look at Belgian primary school teachers competence in math. Verschaffel (2005) already expressed serious concerns on the level of mathematical knowledge of primary education teachers. However: teacher competence did still rise as expected during the teacher training, the people who started teacher training just started at a lower level. Since 2005, the competence in math in primary school teachers has continued to fall of a cliff.

So this is my interpretation:

Really, it's a simple case of lower input quality leads to lower output quality, on all levels of education. It's a cascading effect.

Cascading effect 1 : pupils are instructed by less competent teachers (in maths) so you get a lower level of education in math, which leads to less competent teachers which in turn leads to less competent pupils and so on.
Cascading effect 2: Why was the level of teacher competence allowed to drop? Because less people want to become a teacher, so we are in 'all hands on deck mode'. Which in turn also lowers the quality of the education bachelor and thus it's status and we have once again a cascading effect.
This very soon becomes a race to the bottom, because desperate tries to raise the level of education will very often only lower it.

This has also been my experience as a mathematics teacher in the first grade for about 7 years now. The level of incoming pupils gets lower and lower, making my job harder and harder trying desperately to not lower the bar. I has lead to me looking for a way out of education, further contributing to the problem.
When bright students at 18 years in our school think about pursuing a bachelor in education, we will often recommend they first try something 'harder' and if they still want to teach when they have their first degree, to just do a shortened teaching training of a year. Most teachers in our school have had a very negative experience with the bachelor of education, being mostly stupid busywork at a very low level. That's anecdotal of course, but I think it really sums up the core of the issue.