Because she's just not going to win. She's currently polling in the low single digits nationally, she won't be included in the debates (at least not the first one), and she's on 44/50 state ballots. It's far too close to the election for her to start gaining ground in the polls.
Pretty much every third party candidate is at a disadvantage, even the most popular ones. Take Ross Perot for example - an independent candidate who ran in 1992 and 96. In March of 1992, eight months before the election, he was polling at 21% - 14 points behind Clinton, 16 points behind Bush. He went on to win nearly 20 million votes in the general election and still won zero electoral votes.
Let's compare this to Stein. Eight months before the 1992 election, Perot had 21% support - and went on to win zero electoral votes. It's less than two months before the 2016 election, and a poll released today shows Stein having 4% support. Guess how many electoral votes she'll end up winning.
Not only that, but she's really not qualified to be president. One of my many issues with Trump is that he's incompetent, because he has no political experience. The same can basically be said for Stein. The only experience she's had is being a member of Lexington, MA's city legislature. I would be uncomfortable with either of them being commander in chief and dealing firsthand with other nations with such little prior experience. Hillary, on the other hand, was Secretary of State. Whether you agree with her politics or not, she undeniably has actual foreign policy experience. Being a member of your city's council doesn't prepare you to be President like being Secretary of State does.
And she's also losing to Harambe in the polls.
