Your data is inaccurate because SEDE doesn't account for deleted questions but anyway, there are few factors worth thinking of.
In the past, Programmers were getting a reasonable mix of good and bad questions related to careers, software recommendations and code reviews. Even when topicality of these was slippery, well presented questions about interesting, real problems were considered worthy of upvotes by many readers.
Nowadays, we lost a good part but bad part remains with us.
I mean, users capable of asking good questions in above topics, eventually find their way and stick to specialized sites - TWP, CR, SR.
- What is left to us are askers who want to ask but can't make a good question. They post (or cross-post) their stuff at Programmers, Stack Overflow, Yahoo Answers, anywhere, in a hope to find a kind soul to feed them. This contributes to lowering average quality of questions in these topics.
Another stream of low quality questions are those from users blocked / warned at Stack Overflow.
In the past, they could pollute SO with their low quality stuff for months or even years but for last year or two, due to improvements in burning down SO close queue, they started hitting question blocks and warnings more and more.
- Per stats, Programmers appear to be most popular of SE sites among this category of askers. After being blocked or scared by warnings at Stack Overflow they simply try to dump their low quality stuff over here, thus also lowering average quality level.
If honestly, I can't see what we can do to help with the quality of questions like above, nor how we can control these in review queue.
We can't realistically expect askers of bad questions to "refocus" to specialized sites because at these sites, such questions seem to be detected and rejected even faster than over here. This is probably because of clearer, narrower specialization regulars at these sites build a better sense of what is good and what is not (at least that's how I feel at TWP).
I also can't imagine how it would be possible to make users blocked / warned at SO stop their attempts to leak through Programmers either. I can't think of a way to tell someone facing a block message at SO that they shouldn't try their luck at Programmers.
Wait! You've recently been blocked (warned) at Stack Overflow. Consider that quality requirements at Programmers are similar. Also note that Stack Overflow topics are not covered here.
Are you sure that you want to proceed asking question?
Click No to cancel asking.
Click Back to return to Stack Overflow.