Graeme Zielinski offers his explanation:
Part of the reason that you have not heard about the threats delivered to progressive leaders is part of a strategy that recognized that it was to the advantage of Republicans and, during the recall, Walker, to make the whole process look like a shit show. Every minute they’re talking about made-up reports of rocks or eggs being thrown or other phantom threats, they’re not talking about legitimate grievances with the harmful effects of certain policies. It didn’t help that an infinitesimal, but very real portion of otherwise peaceful protesters are assholes who pour beer on Robin Vos’ head.