You have to think about its application in current video cards and monitors. Right now, most of the use that the display port gets is from eyefinity/triple+ monitor users. This represents a rather small percentage of the overall market. Most people only have 1 or maybe two monitors that they use with their PC. These are easily utilized by VGA, DVI, or HDMI. So most consumers don't even need DP, thereby most manufacturers don't need to include it, especially when adapters exist that do the job for them.
Since DP isn't poised to replace HDMI, I don't know where/how far the technology will go. For the moment however, it's gotten a boost from being included on modern video cards. We'll just have to see how far that takes it. I wouldn't count on seeing a DP port on every monitor any time soon.