In addition to the points Deer Hunter makes, in reading a history of the Shuttle-Mir exchange program, it's evident that Mir was built with very different quality and safety standards than any American space project would accept.
For example, when modules were connected in Mir, electrical and ventilation conduits were routed through hatchways to connect them, making it dangerously difficult to seal off a leaking module after the Progress M-34 collision in 1997. This is an incredibly, shamefully obvious design oversight, which could be straightforwardly addressed with more engineering (e.g. a smaller conduit interface adjacent to the hatch, where cables can be quickly disconnected in an air-tight manner), but that engineering would increase the costs.
Similarly, the station's coolant loop was almost continuously leaking toxic ethylene glycol, and cosmonaut crews spent an absurd amount of time and effort chasing down these leaks. This suggests that the materials selected for plumbing were inappropriate, or the design or assembly process was shoddy, or all of the above. Proper design reviews, quality control, and pre-flight testing are all expensive, but would likely catch this sort of problem.