Two reasons: those who have played the alpha and beta versions know that the online functions of the game currently are very problematic, although there is still are relatively small number of people connected to Frontier's server. Image what will happen when the game is released and a lot more people will try to play. (Hint: think Sim City.)I find it strange that there are people who've played hundreds of hours of the online-only alpha and beta versions over the last few months but are now saying they're no longer interested in the game because it's online-only.
Secondly, the offline functionality has been a promised and repeatedly confirmed feature from the early days of the Kickstarter campaign on. What actually angers most people is that Frontier waited until one month before release to come out and say: "Oh, remember that offline mode we promised? Joke's on you, that will never happen."