The story I have heard in a number of places is that the airline pilot unions have always objected to making the IR any easier because, in Europe, the IR has become the hallmark of the professional pilot. The IR has also often been described as the last opportunity to keep undesirable people out of the airlines. In turn, for a CPL/IR holder, the ATPL has largely become an hour building exercise.
In the USA, the IR is just something you can get at the same school at which you did your PPL, probably with the same instructor. Same for the CPL. Over there, the hallmark of a professional pilot is the ATPL which is an extra exam and an extra checkride to closer tolerances.
Also, Europe has lumped a chunk of the jet type rating into the IR ground school. American never did this - to get that you do the ATPL and the jet type rating, which is as it should be.
I don't think there is any great conspiracy (other than widespread uninformed prejudice) to keep private pilots out of the "commercials' airspace". The numbers of private pilots flying around Europe at the relevant "GA" levels - FL080-FL250 - is in any case miniscule and this is immediately evident to anybody who actually does it.