Because Destiny 1.0 was not meant to be sold to us in all these expansions. We weren't meant to get Thunderlord and Gjallarhorn week one. When they decided to chop up the game and sell us parts as dlc, they put all the weapons into vanilla because they probably didn't have any others ready to use in place at that point and figured they'd make more for the "expansions". the problem was that those weapons they thought they would just replace were absolute Destiny 1.0 endgame and so powerful that they ended up being f**ked. and here we are.