I think it's ego investment. So much of their ego seems hellbent on standing out, differentiating themselves from the rest of us "sheeple" by making an entirely empty gesture; and that is exactly what it was, an empty gesture.
As Kam pointed out, it's hard to do the actual work on the ground. Organizing, canvassing, making calls to potential constituents, talking with your politicians, attending meetings such as City Council and other public events where your voice can be heard, etc... I'm guessing that despite his so-very earnest belief in Stein he didn't do a single one of those things. I bet he hasn't for Bernie, either. If making a third party viable in this country is the goal, where is the work being done by such people
So, again, rather than do the hard work he has a brilliant idea. "Let's just take over one of the parties". We can use their existing infrastructure. When that appears to be failing for the second time in a row, our little boy is indignant so he starts tossing out insults accusing others of not caring about others (which would be laughable if it wasn't so insulting), being stupid and uninformed, doing what others tell them to do and just not understanding how so very right he is, if only we all would just understand that.
We have others here who have an interest in third party candidates and I believe have actually done work to forward their goal. But, guess what? That person saw Trump and despite their location being a lock for HRC, voted HRC. Not our little hero who fancies himself the "one true voice". Despite the admittance that his state was not "safe" he did what narcissists and other people who have no sense of community do, he voted to puff his chest and act for his own ego rather than the good of the country and most especially, its people.
The writing was on the wall with Trump. There was no denying what kind of president he would make. The man has a history. A sordid, seedy, racist, vengeful, childish, ignorant and narcissistic history. To refuse to acknowledge that in the service of an ego and even more, now that not only was just how bad he would be predictable but has played itself out in real time and much worse than thought over the last four years while real, actual harm and injustices are being committed upon both citizens and non-citizens is to be devoid of conscience.
What the solution for a little king, in his mind? Double down. Deny, obfuscate facts, play the earnest little soldier on message boards where the denizens are not exactly welcoming such displays of ignorance. Praise yourself as being a lone voice in the wilderness, pass on Russian propaganda, insult the intelligence of every person who has to read the drivel and of course, play the victim, while denying the truth of the victims being created every day under Trump.