A few reasons why:
- Middle Earth was Tolkein's Christian critique of Europe's pagan spirit. In the Lord of the Rings victory belongs not to martial valour, which is hopeless, but to faith and hope. In Rambo the Christian aid workers are the hobbits. Their illusions are shattered and they are saved by pagan military heroics, which are effective. It rebuts Middle Earth.
- It is brutally violent.
- The real heroes are overwhelmingly amoral in outlook and attitude but effective and on our side.
I'm not saying it was intended that way. I think that it is entirely possible that the film was intended to send a Christian message. However, the pagan attitude creeps in and has a certain plausibility to it. I sometimes wonder if we don't do enough to understand our Northern European/Germanic roots and how they influence our outlook.
Update: I've set this out in rather more detail on CentreRight.Com