I'm guessing it's the same reason a huge majority of laptops still come with VGA connectors--so that people can hook them up to the hundreds of thousands of projectors that are already installed in conference rooms around the world. Switching to a digital signal (either DVI or HDMI) would require extra equipment, since analog cable runs can go considerably farther (and with less issues) than digital ones. Think of it this way, for your standard ceiling-mounted projector, the cable probably runs between 30-50 feet. For an analog cable, you generally would only need a high-quality cable that fits the length. For a digital signal, it will begin to degrade within that cable span, so you'd need to add things like repeaters just to even get the signal from the laptop to the projector. This quickly adds cost for very little real-world benefit.