It's perceived as 'nanny-statism-lite'

People coming here get plenty of warning to watch out for those who are less friendly and then they are left to their own commonsense to try and work out who they think are nutcases or less credible posters. Sorta like the cold hard real world. Also, people with valid messages to impart may be ostracised for no good reason. Longtime posters often use anonynicks to say what they gotta say, and their credibility factor may be high but their vote would likely be low. Also, most of us here probably don't want to make newbies feel bad before they've got a decent 'confidence vote' on their side. Also, if a person has a low confidence vote, that can stigmatise them so that they don't feel as though they can change and grow.. sorta like the cold hard real world too. Also, unless you wanted to refer people back to an earlier thread it would probably require a more complex script behind the scenes, which the people working here are generally against for security reasons. And someone would have to do that extra work as well.