So, why would people colonize Mercury?
Asking that sort of question for any science fiction colony that isn't a duplicate of Earth does promote resistence. For science fiction writers, I suspect, their first answer is, "because it's there! ... Fictionally, I mean... You know what I mean!"
But in terms of good world building, good science fiction should at least offer hints about why people chose to be where they are. Yes, we can get away with a lot. We have colonized all sorts of inhospitable places (Svalbard, anyone?). Sometimes we have no choice. But quite often people find places and figure out how to make opportunities in them, and then the children born there think of their inhospitable rock as home and long for it when they're away.
I picked Mercury as the location of The Sun Runners after being blown away by the worldbuilding in Kim Stanley Robinson's book 2312. His idea was to keep the main city of Mercury, called Terminator, on tracks, and use their expansion from the heat of the sun to push the city ahead of the dawn line. This put some ideas in my head, and I played around with the concept, giving Mercury more than one city, and making them move more actively along their rails -- even reversing direction when they had to. The towering city of Terminator became a more streamlined ten-kilometer long train.
Yeah, my railfanning probably had an influence, there.
Still, this answers the how, but not the why. Why would anybody want to colonize Mercury? How do you make an economic case?
Mercury is composed of 70% metals whereas' Earth's composition is 40% metal. What's more, Mercury has a particular large core of iron and nickel, and which accounts for 42% of its volume. By comparison, Earth's core accounts for only 17% of its volume. As a result, if Mercury were to be mined, enough minerals could be produced to last humanity indefinitely.
Its proximity to the Sun also means that it could harness a tremendous amount of energy. This could be gathered by orbital solar arrays, which would be able to harness energy constantly and beam it to the surface. This energy could then be beamed to other planets in the Solar System using a series of transfer stations positioned at Lagrange Points.
Also, there's the matter of Mercury's gravity, which is 38% percent of Earth normal. This is over twice what the Moon experiences, which means colonists would have an easier time adjusting to it. At the same time, it is also low enough to present benefits as far as exporting minerals is concerned, since ships departing from its surface would need less energy to achieve escape velocity.
Lastly, there is the distance to Mercury itself. At an average distance of about 93 million km (58 million mi), Mercury ranges between being 77.3 million km (48 million mi) to 222 million km (138 million mi) away from the Earth. This puts it a lot closer than other possible resource-rich areas like the Asteroid Belt (329 - 478 million km distant), Jupiter and its system of moons (628.7 - 928 million km), or Saturn's (1.2 - 1.67 billion km).
For many, the reason why we go somewhere is usually because it's there. For many more, the fact that there's money to be made is a good side benefit.