There are a few reasons for this, the main one being the location of the incision.
There are multiple ways of approaching a hip replacement but all of them result in the incision being 'out of the way'. A knee replacement has the incision straight down the front of the knee. This has an effect on the rehabilitation.
To prevent the knee from stiffening, the knee must be quickly exercised. This means bending the knee and stretching the skin around the incision. This is responsible for a lot of the post-operative pain. After a hip replacement, you are not required to do movements that stretch the scar, in fact you are told to avoid them.
This is why I tell people you can be a little lazy and do OK after a hip replacement, but to get a good result from a knee replacement, you are going to have to work. The pain is something that gets in the way but should be seen as a temporary obstacle to years of pain free walking.
You should be going into a knee replacement operation focused and motivated. It will be a few weeks of toil for the greater good. I have always found that those who have put up with the pain of an arthritic knee for years manage with ease. At least this pain is because of an operation that is going to help. That is much easier to deal with than the pain of arthritis that gains you nothing.