It's simple, people will use the most viable weapons they have access to in order to keep up. Every time a weapon gets nerfed, the list of viable weapons becomes smaller. So when weapon A gets nerfed, weapon B becomes the next viable weapon. When weapon B gets nerfed, weapon C is next on the list. And so on. For example, MIDA has always been a good weapon. It just never got the attention that Vex or Suros Regime had. Back in the vanilla days, people would use MIDA to compete with Suros. Now the day has finally come that MIDA is now at the top of the list.