Why should she? Her party represents different interests to the Democratic party. In a fair and just democracy she should be allowed to run for office to represent that platform. If the Democrats have an issue with that, then they should adopt some of those policies. For instance, one of her policies is an anti-genocide policy. This might seem a bit radical and extreme but it's apparently very popular. If the Democratic party stopped being pro-genocide then there would be less threat from Stein.
People really need to stop acting like the Green Party just came about in the last decade to spite the Democrats. If the Democrats had a decent platform they'd stop losing votes to them.