Christian Rudder's OKTrends blog (and Sam Yagan's presentation at their acquisition celebration) even spelled out the reason why: some women on OKC (or, more rarely, men) would acquire the "Replies rarely"/red color on their profile, for almost never (<10%) replying to initial messages, which was generally considered to be undesirable behavior, even in the negative (there is value in a negative message: "Thanks but I'm not interested due to age/location/other factor". And also OKC could then measure whether users' stated preferences mismatched preferences inferred from which set of users they message e.g. people who say they're looking for 30-55 for LTR but tend to message people 21-35 for short-term). And before anyone points out that younger more attractive female profiles would get more initial messages than males (up to 200:1 more), OKC used to allow you to set filters on the other user's age/distance/other criteria, so you could automatically filter those out. Also, factor in the usual caveats that many users on dating sites tend to lie about their age/weight/height/location/status/etc.
Anyway, to avoid getting labeled the dread "Replies rarely", some (mostly female) users got in the habit of sending one-liner responses that were ambiguous/non-committal/cryptic/negging. And then not responding further (but without unmatching, which only took a single click). This was making their profile look less undesirable but generating pointless message traffic and reducing the overall utility of the platform at actually attempting to match people (for compatibility, not just initial attraction). Hence, OKC tried to actually measure initial exchanges to figure out which ones led to genuine back-and-forth conversations of 3+ messages (which is an ok proxy for inferring a match, certainly a better proxy than just counting initial messages/likes/votes on photos). Yagan jokingly referred to this as "Every Monday morning, we ask 'How many three-ways did we set up over the weekend?'"). (PS, Rudder and Yagan both stressed that users' names/identities/ identifying characteristics were kept out of the analysis.)
After the 2011 IAC acquisition, most of this platform quality control (and looking for constructive insights) went out the window pretty quickly and the three cofounders moved to OKCupid Labs. But it was good for the brief while it lasted. By 2013 a chainsaw had been taken to most of OKC's unique features, esp. for free users.