Because in all honesty, the gally NEEDED to be made useless. If bungie didn't just leave old gear in the past, it would become even harder to store our gear and we would have more people bitching about vault space. No matter what gets left behind, people will bitch, but in the long run, bungie made the right call!
Edit: don't get me wrong, I am sad to see OTHER exotics go, but I think gally was, at least, the right choice. Hard light was unobtainable for me, and when I got it after 30 weeks of grinding, I was sad because of how useless it had become. I want the buff they are giving hard light to not be in vain.