Was it ever explained why Space Station V, used as a transfer point from Earth orbit to the moon and other planets, was in a low Earth orbit rather than a higher orbit, which would have been more efficient for transfers to other locations in space?
Low Earth orbit is the most efficient place to locate a transfer station. The energy required to reach low Earth orbit is the most efficient place to locate a transfer station. The greatest difficulty in launching spacecraft is getting them into stable orbits around the Earth. The reason for this is that the energy required to reach the initial low Earth orbit needs to be imparted all at once, starting from launch. In contrast, once a body has reached orbit, it is comparatively straightforward to launch it further out into space.
The is seen historically in the NASA program. The first two manned spacecraft programs (Mercury and Gemini) were entirely based on low Earth orbit operations. The Apollo spacecraft headed to the much larger elevation of the moon operated by essentially blasting into a trajectory that would lead to a low Earth orbit, then following that up with another burn that carried them on a free return trajectory around the moon. The first stage of the ascent was, by far, the most demanding, due to the Earth's huge gravitational field that had to be overcome.
The same kind of situation applies in a hard science fiction context as well. Once systems are in orbit, they can be assembled to make a further craft at leisure.
Space Station V hosted a range of facilities including an orbital Hilton Hotel. It was used as a transfer point, sure, but the majority of people on board seem to tourists (who are presumably traveling from Earth to the station, then returning to Earth) which means that a nice close Low Earth Orbit would be a far more efficient location than a high orbit in the middle of nowhere.
The name "V" (five) also suggests that there were at least four other stations, possibly in different orbital positions depending on the need for various transfer windows. Floyd simply traveled to the nearest station that had an available shuttle.
No matter how many times you left Earth, Dr. Heywood Floyd told himself, the excitement never really palled. He had been to Mars once, to the Moon three times, and to the various space stations more often than he could remember. Yet as the moment of takeoff approached, he was conscious of a rising tension, a feeling of wonder and awe - yes; and of nervousness - which put him on the same level as any Earthlubber about to receive his first baptism of space.
2001: A Space Odyssey - Arthur C. Clarke
The transportation model in "2001" consisted of reusable winged shuttles from ground to a Low Earth Orbit (LEO) station, then transferring to LEO-to-lunar-surface shuttles.
This kind of model was prevalent in mid-century sf; sometimes a Lunar orbit station and moon-to-low-lunar-orbit rocket shuttles were added (Heinlein favored this system).
This thinking was based on the plans of von Braun and others. It was thought more efficient to use specialized craft for each leg of the journey rather than one multi-purpose ship.