It's simple.. Because by not being able to use year one weapons which people enjoyed so much, it forces the players to play more and grind for the next best replacement.. Which could take days, weeks or months and the longer you play the more unlikely you're going to want to give up having out so much time and effort into what you've managed to achieve. Bungie don't care about its community beyond drip feeding it enough to keep people grasping for the little bit more to satisfy their need for the next best thing, which in the end as we all know is empty and void anyway, hence why the need for that 'special' weapon or armour never ends, because it was never really THAT great to begin with. Bungie is like a slavedriver who feeds his slaves just enough to keep them alive and able to work. And this community for the most part (including myself for a while) are like deluded slaves who think the slavedriver feeds them because he cares about them when in actual fact its because just about keeping them alive so they make him money. TTK is the end of the line for me and I suspect for a lot of other people too.