Because the devs and Activision wanted to dumb down the story so it catered to a wider audience to include teens. And the lead director didn't like how it was too linear of a story so they chopped it up, spliced it together and changed it to a mission styled game. The writer for the game actually ended up quitting/was fired just before vanilla destiny. That's why there is so many inconsistencies in vanilla destiny. Everything up to and including the taken king expansion was alrdy on the disc, it was supposed to be part of the main game. Activision wanted it broken up because they saw a way to make money. They thought it was "too much content for one story". You were supposed to be rewarded with damn near every exotic just by playing thru the story. Gally, an arc hard light, suros, all of it.

Thunderlord on the first mission. That would have been...shocking to some. [spoiler]I'll see myself out.[/spoiler]