It's just not popular enough to be handled by bot scripts (yet)... and most of spam bots are not targeting one particular site so this worked for a while, just

It's just not popular enough to be handled by bot scripts (yet)... and most of spam bots are not targeting one particular site so this worked for a while, just like math captcha and drag-an-image worked for a while. The moment they get popular enough for bot authors to tackle the issue, they'll stop working.