Why is the book of Psalms so rarely preached on, and why is there a dearth of good commentaries on this treasury of poems?

I believe it's because in our fast lane culture we have lost what it means to meditate. More than any book of Scripture, the collection of Psalms are best mined and most deeply drunk from through long meditation and heartfelt praise and tears. 

The psalms speak to real life like no book in the Word. But to get there it takes more than analytical tools of Bible study. It requires mental marination in the heats and chills, ups and downs of real life.