Oh I was talking about why the streamer changed it... not why people copied it.I don't think this explanation works. Yeah, maybe it's why the streamer changed the deck. But people copying the streamer's deck don't need to change if the point was to make it harder for people to tell what the secret is - the streamer's problem is that people know what his or her deck is. Some random player doesn't have this issue, and in fact can benefit from having a deck which is different from some popular deck but which can be mistaken for it at first. Even if the random player isn't using Counterspell, people will think their secret might be Counterspell because they otherwise appear to be playing the streamer's deck.
People copy because people copy. No real reason for it other than following the masses.