Not sure how much this helps.. but I also remember reading that one of the safety protocols built into the Iron Man suits is that it only responds to Starks thought patterns and brain waves.
That is why Natasha, in Iron Man 2, made a somewhat sarcastic remark in the Diner with Tony and Fury about how there are supposed to be security safeguards to prevent unauthorized access to the suits. Tony had already programmed the suits to respond to Rhodes brain waves and other biometric data which is why he was able to take it.
I would postulate that, because this advanced security method, which requires the brain scanning and mind reading technology that only Stark has, is why they could only modify the suit that Rhodes was already programmed to access, though by Iron Man 3 it's possible that others were able to figure it out or work around it.
I also thought that the lack of the miniature Arc reactors was also a stumbling block to anyone else being able to create or use the suits as well.
Even if they were able to physically recreate all of the technology, without a similarly advanced operating system such as JARVIS, it's likely that they were unable to get it all to work together as a cohesive unit, or work as smoothly and perfectly so that it responds to the wearers thoughts and desires as does Starks Iron Man suits.