There are several issues that come into play when integrating payloads to boosters. For example, the Shuttle has a fairly gentle ride to orbit. Some payloads might have to be beefed up to handle higher g loads and vibration levels. Other issues involve electrical interfaces and things like that. Some can be easily addressed, but others can require extensive (and expensive) modifications.

For ISS payloads, there's a much bigger issue - components designed to be carried on the Shuttle lack propulsion, guidance, electrical and TT&C systems. The Shuttle can carry those components directly to the ISS, so the components can be a lot simplier. Unless we can develop some form of tug capability, putting those modules on an unmanned booster like the Proton or Ariane V could leave them in orbit, but no where near being able to dock with the ISS. All of the space station modules launched on the Proton in the past had the necessary systems to fly on their own and dock with the target station.