If I'm getting it right then you're asking that why trending tags should be regular instead of going away after some time. I believe it has many reasons. The first is that these hashtags are sometimes used for new upcoming events. So when those events are over then they become obsolete.

Another reason is that humans have short attention span. And any interesting thing become less interesting after a while. Look at ice bucket challenge. Everyone was crazy back then about it. It was a silly thing but then it lost its silly charm after some time. Initially people were getting dopamine bombardment from this silly activity but once it became regular then it became less impressive & thus hashtag gone. So yes. Something comes & everyone become crazy about it & then people pack the activity to move on to sillier things.