Looking at the modules of the ISS every one of them (the Russian ones in particular) seems to be covered by silver and white cables, wires and pipes on the outside. Some of them have been added later probably to connect the modules. It makes me wonder why these connections are not part of the docking port inside or at least covered.
I dislike writing an unreferenced answer, but I don't believe that a reference saying concisely "It is designed this way because of this reason" actually exists.
So I'll give you the "conventional wisdom" which I gathered from having worked with ISS space shuttle assembly missions from the first one to the last one (but not every one) and a shuttle/Mir mission.
The US ISS pressurized modules connect to each other with Common Berthing Mechanisms (CBMs). After the modules are connected, the CBM electronics and cables are removed, and the modules remain bolted together through a ring-shaped interface. Inside this ring-shaped interface is a squarish hatch. This is well shown in this picture...
... which I took from this excellent q&a How are hatches set up in the common berthing mechanism (CBM)?
Given this setup, you have a choice of three ways you can run the electrical, fluid, and data connections (not necessarily exclusive choices) from one module to the adjoining module.
- You can run the connections through the hatch openings. This was the design used on Mir
(image source images.nasa.gov download)
- The good: It is the easiest way to set up.
- The bad: It clutters up the hatchway.
- The ugly: When you get a leak in the station and need to close the hatch fast, it is bad news. Not only does it slow things down, but you sever all the connections.
- You can run the connections in the pressurized space between the rim of the hatch and the CBM
- The good: The cables, hoses, and pipes are in a nice pressurized environment. The crew can connect them up to the adjoining module without going EVA. No problem closing hatches in an emergency.
- The bad: There are lots of cables, hoses, and pipes. They would take up volume in the ISS, and having to run them along the walls would change the design of the modular racks significantly. Imagine running all those connections though a Node with its six hatches.
- The ugly: Every cable, hose, and pipe penetration is a potential leak point. Remember, there are lots of cables, hoses, and pipes... The modules were launched pressurized, during assembly the interfaces would have been exposed to space for years in some cases...this is to be avoided.
- Run the cables, hoses, and pipes externally
- The good: No problem closing hatches in an emergency. No use of precious module volume.
- The bad: Cables, hoses, and pipes exposed to micrometeroid/vacuum/general space environment
- The ugly: To connect them up or work on them, you have to go EVA.
The ISS program weighed these and other factors I don't know about (cost, weight, schedule...the aerospace holy trinity) and chose approach 3.