Why isn't the question closed?
Because both you and I — as well as everybody else on the site — has chosen not to cast a close vote. The moderators, though they do occasionally preemptively close question, tend to leave those decisions to the membership at large.
Even as someone who left an answer to the question, I agree with you that it clearly meets the criteria for "opinion based" questions that would otherwise be closed. But my feeling also is that this question has clear and specific value to the site and that while answers are opinion based, they also are experienced based, and by having a variety of them, different readers may find different solutions suitable to themselves.
This is stretching (at least) the spirit of SE sites, which are intended to be quick and clear Q&A posts. But our specific site is inherently opinion based, dealing as it does with questions of musical interpretation, for example. So there is an ongoing tension between which posts cross the line between okay-opinion-based and too-opinion-based.
If we believe these answers are valuable, why are they not upvoted?
Lack of voting is an ongoing problem on the site. Moderator @Dom has brought this up a number of times. I can't help but wonder, in this case, if the sheer number of answers, and the length of those answers, is a deterrent from even reading them in the first place. That would certainly be another good argument against this type of opinion-based question.
But to the larger issue of voting, consider the five members with all-time reputations above 50,000. There are five (myself included), and on average we collectively post 65 votes per month. That means of the five of us, we average about two votes each day.
As I write, Data Explorer reports 83,613 total posts (questions and answers) on our site. The site is a few days past its 11th birthday. So on average over the life of the site, we average 633 posts per month since the site was established, or about 21 posts per day.
So the 5 most prolific users (by reputation) collectively vote on one of every 10 posts.
A total of 378,559 up or down votes have been made since the site's inception. So the average post receives 4.5 votes either up or down. There are 61,357 users registered who collective make those 4.5 up or down votes per post.
Restricting that to users with reputations of 1000 or greater, there are 393 of us who have received approximately 100 upvotes to our own posts. We post, at the most (i.e., if no other users vote ever), just over 7 up or down votes, collectively, per month.