Look in the end of the c”c in the last tziurim. I unfortunatly cannot remember which one although I believe it is the third that disscusses a little bit about shiduchim.
Shkoyach, the reason is because you are allowed to discuss information for a toelet which includes only for the good absolutly no sinah/hatred along with no additions or subtractions and, we know, lifnei iver to titen michshal/before a blind person do not place a stumbling block hence the halach listed above.
You cannot speak to ones enemies regarding a person. Additionally, if you accidently say, “but I heard he/she is a ben/bat Torah,” I don’t think I need to complete this to tell you this is avak lashon hara or even perhaps l”h itself because we cannot praise people before their enemies.
Not only that, you may become caught up their emotions and loose the fact that you are speaking for a specific toelet and possibly believe such falsity, anger and cause accusations in shamiyim endagering the whole klall.