It's a consequence of our voting system.

If two parties that have similar ideals (call it A1 and A2) enter an election with another opposing party (call it B1), benefits the opponent, because people who agree with either A1 or A2 can only vote for one.

You can end up in a position where 30% of people vote for A1, 30% for A2, and 40% for B1. Because A1 and A2 were so similar, neither of them can achieve plurality, despite their message itself being more popular, and the race goes to B1.

Btw, back in 2000, the Green party got enough votes in Florida that, had they instead voted for Gore, the whole "chad" debacle would've been a non-issue. I guess Gore wasn't environmentalist enough for them. Bush/Cheney ended up winning...and they certainly were not environmentalist in the least.

I credit this for being the historical turning point where everything really started going to shit. Bush v Gore and the chads. And thus, to this day, I'm a tree-hugging hippy who will gladly say: fuck the green party.

...To take it a step further, in order to attract centrist or opponent voters, the party has to be moderate/centrist. It cannot be exceedingly progressive, or it will lose its more moderate members.

In essence, the voting system causes two party systems, and the "big tent" party causes the big tent to suck and always move opposite the driection it really should.