Part of the reason is bosses now have more mechanics and less health. It wouldn't be fun if u killed them in 5 seconds with a mega powerful year 2 gally. Although many exotics are not and will be brought to the new year. There are other reasons also but I won't go into them. Just short reasons. It is IN SOME WAYS an mmo. They don't want people using the same weapons all the time because all dlcs would be obsolete.