It's all about listeners' beliefs about what staying true to artistic integrity means. If a record was recorded mono (mostly very old recordings), and then re-mixed or re-mastered to be stereo, it means that someone other than the original artist or engineer has altered the source material. This concept is contrary to some people's desire for the original artistic intent, where the desire is for all of the imperfections of the original recording to remain intact.
As for classical records, if they're in stereo, they were likely recorded that way, rather than digitally enhanced after the fact. Classical listeners generally prefer an experience where the recording process is as noninvasive as possible, and hope to gleam as much detail as they can from the performers, rather than the recordings. Classical recordings generally have minimal post-production for that reason. Stereo recordings offer more of those details, and it's a truer representation of the experience of the performance.