Because blizzard sees the game population shrinking and they are trying to compress the social space in order to maintain critical mass of player interactions. It is why they did the leveling revamp. Why the tried to stamp out on 110/111 twinking in early bfa, separated out twinks in their own bracket, why they don’t allow you to twink 51+. Why they disabled legion legendaries. Why they are putting all the holiday rewards only for level 60s.
They want everyone at 60 because they believe that people falling off in the process beforehand is detrimental enough to impact the overall game population.