Because your playstyle contradicts what is the norm. Players who have 1 main.
If you can keep 4 characters fully equipped, what it means is that someone who plays as much time as you, but with only 1 character, runs out of things to do. That was what happened in Cata and the end of WOTLK. there were just too many players who had a real main who had nothing to do with their characters to advance them. The vast majority of players fall into that category of only having one main.