UK uses left-hand traffic. Traffic flows on the left side, people walk on the left. Is there any reason to stand on the right while using escalators?
Why do people do it today? Because of the multitude of signs that say to do so!
If you're asking why it started, this BBC article (includes video) holds what may be the answer...
(the video also includes clips of 1920s tube-escalator-ettiquette cinema humour such as a soldier tripping up at the bottom of an escalator because he's drilled to always march left foot first)
I moved to the UK for 4ish years back in 2007. In advance, I read the Wikitravel and Wikipedia articles to prepare a bit, and in one of them it mentioned that you stand on the right, and walk on the left. And I laughed and thought - 'no way would people actually follow that'.
Then I got to London, got my first barge done quickly when I was standing on the wrong side. I was amazed - could people really get upset about it?
Soon enough, and sure enough, it was me getting annoyed when I come across someone on the wrong side, and I'll admit to a bit of barging occasionally too. It's funny how quickly you adjust.
Now, as to the why? I also wondered at first - if you drive on the left, why not walk on the left?
And then it hit me, that's exactly what you're doing. Escalators, remember, aren't meant to substitute for walking, but to help. And you walk on the left, up the escalator. It's when you're NOT walking, that you stand on the right.
And that was how I rationalised it. After that, it worked fine in my head.
Now that a system is in place, as you mentioned it needs to make sense to others. Australia, for example has the reverse. And thus all the signs you'll see all over the tube - 'keep to the right', 'walk on the left' and so on.
And so society continues on.
You're asking the question wrong.
"UK uses left-hand traffic." does road traffic sit stationary? No - it moves. We travel on the left. Period. If we're on the road, we go on the left. If we're walking down an escalator we walk down the left. However not everybody wants to walk (travel) on the escalator, so they stand aside (that being the right hand side), so movement can still carry on. The question is why do British walk on escalators when they could stand? But that's another question ;)