Because Obsidian doesnt have anything to show yet, and even if they would have WIP-screenshot I wouldn't want them to show it because it would most probably just backfire due vast majority of the people being stupid and not getting that it's just WIP.
Not a solid argument.
The original presentation on KS was underwhelming. That clip was not enough. There would be no backfire if they had shown some artworks from the start, the skills systems, perks, classes and other things. But in the current situation, they basically dug themselves into a hole.
Each update can start a shitstorm: cool-downs, bland artworks, magic system, RTwP and so on. And now, they are basically afraid to show anything, because they don't want to lose money. A behavior which is not only unethical, but it also questions the entire vision behind this game. Sure, it makes sense from a financial point of view, but in the end, the idea is that: KS is a platform based on trust and Obsidian should show more trust in their fans, instead of clumsily chase the money.
As it has been said in here so many times, Obsidian clearly has paced out the info they give on the whole month so they have something concrete to update on, such as classes just the other day. Giving information out gradually also helps them to keep Project Eternity in the news-sites when they potentially write news-stories on the developments and gain more visibility the kickstarter project.
They might not have specifics even down yet before they know how big budget they get so they wont have to re-do the designs, such as how many classes and races they can do, or what kind of combat/magic system they can have (as for example how many abilities and spells they can code into the game) - system with fewer spells works well with some other magic-system, but more spells needs different kind of system.
They are in the very early of the development and I completely understand why they chose to run the Kickstarter campaign now, instead of getting something ready - if they'd had designed the story, plot and scope of the game for say, 2 million dollars, and they would get 3 million dollars, they would have to at least partially redesign the game and it would take unnecessary time and money to do, and obviously manhours.