How would this work with the direction the story was going? We still need to go to the Borealis in the frozen north, find Dr. Mossman, and find out the whole portal technology connection with Aperture.
Years ago someone on another site proposed that Half-Life 3 take place on the Combine's home turf. That at the end of the HL2 arc we teleport to their world or are sent there by the G-Man. I could see an open world working there, but with closing out the HL2 story it simply doesn't fit.
Also, Gabe mentioned in an interview that looking back on HL2 he felt they announced it too early, to him it felt too incomplete and the Sept. 30 2003 release date was too ambitious. He learned his lesson with that so I think that is feeding into why we haven't heard much on the game thus far. He says it's coming and I believe him, but I'm taking this article with a handful of salt.