The reason varies. There are many HDTV transfers were created long before there was a blu-ray release. In some cases, the HD transfers were created when the DVD release was being remastered so that they would be ready for HDTV or HD video when it was available. During the time that HD-DVD and Blu-Ray were duking it out, a few companies created initial transfers and waited to see what format would win out. There were also HDTV transfers that were created straight off the film or negative without as much processing because they were created for HDTV and there was no video release confirmed for the immediate future. With TV broadcast, a lot of presentation standards are lower than those of a video release so DNR, EE, color and contrast adjustment, etc... are not used as heavily. When a film is released on video, these "enhancements" are generally made to try and make the film look more like modern releases so it is more attractive to casual viewers.