There's really no logic to this - it's just common usage.
However, in most cases, we used to use the "lit" form when we couldn't use the "light" form as the infinitive, e.g. you can't sunlight something.
You can, however, highlight something, therefore that thing is said to be highlighted. You can, of course, spotlight a stage and floodlight a pitch, so this distinction is effectively lost these days...
It seems as though lit is used when it is prefixed by the thing that does the lighting, sun, moon, candle &c. This is not the case with highlight or footlight.
Thanks for all your replies. Looks like it's just a usage issue as MarkRae says.
I was just intrigued as to why highlighted seems to buck the trend. As for uplit, I have certainly heard it used, in the theatre when a subject is lit from below for example but you're right, I can't find it in any dictionary so I'll delete it from my vocabulary!