Why Are Mono LP's So Undervalued By Most Classical Collector/Listeners?

Yes, imaging is more apparent with stereo. With mono, its more subtle. I tend to concentrate on featured artists and their instruments like a Gould sonata or a Heifetz concerto. Some of my Glenn Gould Columbia mono’s are just "in the room" for me. With these type of recordings. it's more of a "what" than a "where" experience for me. 

The Julie London monos are spectacular at capturing the sense of warmth and intimacy of her lyrics. BTW, I think she slays Diana Krall.

Tablehockey.
I actually have that exact Julie London album in mono.... It does not get much better for female vocal.

Was just digging out mono recordings the other day to try with a new phone stage that also has a mono switch, can't wait to try them and the phone stage with another arm wand with my Dynavector DRT XV1s that I use for my mono recordings even though it is a stereo version, have used it that way every since I read an article by the DRTs creator challenging owners to try it that way and have to say that it is really good, so good I never sought out a mono cartridge for the task, also only have maybe 100 mono albums. Looking forward to listening to them this weekend. Enjoy the music

The monetary divide between mono and early stereo issues on the collectable labels - columbia SAX/33SX, hmv ASD/ALP, Decca SXL/LXT, etc. - is immense. I'm an avid collector of these labels, so naturally I did investigate many of the much cheaper mono's before deciding to invest in the stereo issues. In the case of orchestral music my preference for the stereo is unescapable. The mono's usually have the same gorgeous tone colour, but they simply cannot reproduce the special width and depth that make those early stereo's so very special. So unfortunately there's a 'sound' reason for the price differential.  Hoever, with most solo and chamber music the situation is very different. Samson Francois playing Chopin sounds just as good in mono for a fraction of the cost. Easy choice.