The problem with DisplayPort is the amount of video displays that supports it. Especially those fairly priced.
As display port has basically bombed, they might not want to rock the boat but:
DisplayPort is currently royalty free, while the HDMI royalty is 4 cents per device and has an annual fee of $10,000 for high volume manufacturers.[26] HDMI Licensing LLC claims that, like HDMI, the DisplayPort specification allows for compensation from implementers to unspecified rights holders.[27]
I guess the problem actually comes from trademarks:
"Prior to mass producing or distributing any Licensed Product or component that claims compliance with the HDMI Specification (or allowing someone else to do such activities), each Adopter must test a representative sample for HDMI compliance"
So in theory if you don't claim it's compatible with the HDMI specification then you don't need to test it. There must be ways round it, because from the way it's written then even saying it's compliant with HDMI using a DVI to HDMI adapter would be a problem.
edit... It looks like DisplayPort to HDMI cables are under fire....
http://www.macrumors.com/2011/07/08/minidisplay-port-to-hdmi-cables-unlicensed-and-cannot-be-sold/, hopefully someone will sue hdmi and sort out the mess.