It may not have been "hard" but levelling was objectively harder during Vanilla/TBC, especially vanilla.
Reasons:
Your gear was crap and badly optimised. You often didn't get an upgrade for ages and were using a green weapon
You didn't have class mechanics that allowed you to deal with more than 1 mob - ie warrior 1-30
Your relative power level to a mob was fairly even. In retail you're probably 5x more powerful and that's without heirlooms taking you to 10x more powerful than in classic
Defias Pillagers
Yes, it was more grindey, that's not what made it harder. What made it harder was how much less powerful you were in comparison to the world around you