I'm not sure that's true (that it's empirically observable without bias). The trouble comes from an assumption that by giving up X (that you value only a little) to achieve Y (that you value a lot), that those other people would be better off, when they simply value X a lot more than you do (or Y a lot less). When they consistently vote to prioritize X over Y, you empirically observe some outcomes, apply your own weights to each factor, and conclude they're acting against their own best interests and cite your evidence as empirical proof, ignoring the judgment you applied when determining the weights.
I don't think people exhibiting this bias (unconscious or conscious) are intending to be condescending, but it comes across that way quite often.
Some people prefer to live in a society where the government has minimal power/influence over citizens' daily life (X) while other people prefer to live in a society where government power and influence is strong and consistently exercised to protect the most vulnerable (Y). They're different choices.
And unfortunately a major problem with proportional representation is there's no one single best way to achieve PR - any campaign to achieve PR would be stymied by splitting the pro-PR faction between STV and IRV, for example.
Labour's official position is to negotiate a deal and then hold a final referendum. [1]
So there were three positions in the election (leave, remain, vote again) and leave won the largest share of the popular vote.
The majority of the country doesn’t want the Tories to negotiate the withdrawal. But that’s what is going to happen.
PR would probably lead to even more parties with each having fewer votes and none ever having a majority.
My observation of the BBC's coverage was that while they at least didn't put a positive spin on the Tories, it felt like they were doubling-down on anti-Labour and anti-LibDem negativity. I still don't believe the BBC is in any way biased towards any particular political party, but I feel there's a feedback-loop somewhere in the media system. Once the meme "Corbyn is unelectable" becomes a meme and people don't stop to question it - or it becomes self-referential ("Corbyn is unelectable because the people believe he's unelectable") then we have a problem.
(Disclaimer: I would not actually vote for Corbyn myself - just saying).
How do you explain away people who didn't vote for Corbyn because... they would prefer a non-anti-semitic leader? I have a hunch. You might say, "Well these people were duped. He isn't IN FACT an anti-semite." But, now the goal posts have changed and the discussion moves to whether certain claims are true, not whatever is in someone's best interest.
The thought that people you disagree with are simply fools because they don't know what is in their best self-interest, is not only arrogant, it's patently just not how the world works. Like always, it all comes down to whether your position or their position actually maps to reality. And if you think that you can't be wrong, or that it isn't likely that you get at least some things wrong, then your position is already hosed (not to mention lacks epistemological humility).
Because they want change.
Condescension has nothing to do with it. It's empirically observable that a proportion of the electorate doesn't vote in its own best interests.
It's also empirically observable there's an entire industry of media outlets, think-tanks, journalists, bloggers, TV presenters, radio commenters, online commenters, and other influencers which is paid huge sums every year to make sure this doesn't change.