I think it happens primarily because of influence and ignorance.
It probably happens most often because Christians, out ofthe religiois groups, have the greatest influence on society and government, at least in the west. And people want to know how they see things because of it. It's the viewpoint that they are going to come into contact with most often, and it will be the one that has the greatest effect on their outlook.
And I say ignorance because I think that most people assume that Christians and Jews understand scripture the same way; a co-worker recently told me that Christians are "just Jews who believe in Jesus". And, sadly enough, this is the position held by most people I discuss religion with face-to-face. I don't think it is willful ignorance, but it is still there.