Times have changed. Levelling isn't enough anymore. It's been 8 years of levelling.There was enough for them to do in Classic and TBC. Now leveling is so easy and boring that they just reach max level, do LFR and get bored.
If they make levelling too hard, "serious players" who leave the game due to Real Life™ circumstances can't rejoin their friends to raid and don't bother playing anymore until next expansion