So why do people vote for a party they consider second-best? The obvious answers (stupidity/tribalism/habit/complacency) are both dull and depressing, so let's try and find some more:
1a) Finger in the dyke: My party deserves to lose, but it looks like getting whitewashed. So I'll vote for it to keep it alive.
1b) Balancing act: Contrariwise, I rate the other lot, but not as much as everyone else clearly does. Ideally, I'd like to see them with a majority of <30, so I'll keep my vote against them as a counterweight.
2) Look to the future: My local MP/candidate is part of a group of [young reformers/hard-core traditionalists] who will [breathe new life into this moribund party/get us back to our roots]- so I'll vote to support them, even though I can't stand the leadership right now.
3) Positive re-inforcement: My party's position sucks, but not as much as it did four years ago when I did vote against them. They're going in the right direction, and I need to encourage that.
4) I think the other lot will be better for the country overall, but I'm a strong supporter/opponent of [single issue policy] and so I cannot in good conscience vote for them.
5) General principles: the leadership may currently be a shower of nincompoops, incompetents and backstabbers, but the party stands for a basic political philosophy I want to support.
These positions are not without their flaws: I'm not trying to suggest that loyalism is necessarily virtuous. But voting is a terribly unsophisticated process: we start, ideally, by developing our own nuanced position on everything from fiscal policy to health provision, foreign interventionism to environmental regulation; then we compare competing parties' policies to find the best - or least worst - match; then we reduce this to all or nothing support for one party's bundle of policies. No telling from the X on the ballot whether we're whole-heartedly behind our candidate or favouring them only on balance. No doubt, however, how much of our support they will claim when elected.
Consequently, it's probably no surprise that people are reluctant to switch votes, or try to make their vote the beating butterfly's wing amidst the noise. A fool's errand, given the sheer volume of votes and our inability to predict them accurately. But conversely, it's because we don't just vote for the party we think best that we get nuanced electoral results: massive majorities only happen when once-loyal voters finally break. It's good that there are independent voters who lean narrowly left or right; but it's also good that big political shifts demand the sceptical and reluctant assent of more entrenched voters.