Two main reasons, I think.
First, the FAA IR is a rating which is designed for a PPL to add to his license. It is also a requirement for sitting in the right hand seat of a jet.... but in order to upgrade your FAA CPL to an ATP you need to do a skills test which, as I understand it, is of similar difficulty to our IR. On the other hand, JAR does not have an IR which is designed to be accessible to a PPL - the only instrument qualification (ignoring the IMC rating) is the IR, so may as well make it harder to start with, then the test for upgrading to an ATPL can be simpler.
Secondly, one of the most difficult parts of the IR is NDBs - tracking, flying approaches and flying holds. Understanding NDBs is pretty vital around our airspace, but even so the examiners seem to place a massive emphasis on them, far more than they warrant. Whereas in the US they barely know what an NDB is, let alone how to use one - it's so rare they are used in anger that the IR can concentrate on more practical things without having to worry about bashing your way around an NDB hold.
That's my understanding, but I have no first-hand experience of the FAA IR, so I may well be mistaken.
FFF
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