The people who backed it on Kickstarter have exclusive access until David has shipped all of the Kickstarter copies.
After that, he will sell his remaining inventory of physical copies.
At some point after that, he'll offer the a digital only sale.But he wants to sell down his inventory, first, before he starts selling digital-only copies.
Having said that... if you buy a physical copy, you get the digital version. It was the same way with X2. I bought X2, downloaded it, and started playing it for several days before the physical copy showed up. And I haven't even opened my copy of X3 yet. It's still sealed, with his autograph on it.
Sorry for late reply. But, the above answers are more or less correct. I will offer the digital-only download eventually. I recommend the lite version for people who don't want the boxed version. I really feel that having a printed user manual is essential for playing this game. The PDF is included with the digital download, but it isn't the same has being able to look in the manual while the game is up and running. Most of the people that end up not liking the game don't like it because they can't figure out how to play it well. And they can't figure that out because they didn't read the manual.