>>718991140
You won't ever find a common-ish answer as to why. Some would tell you in TBC when flying was introduced and the overworld became a lot emptier. Some would tell you it was actually WOTK when LFR was made which turned the world even more emptier. Others would tell you Cata with the old zones being gone and/or altered, etc.
Hell, some WC lore purists would even argue that vanilla was the beginning of the end with some wonky decision they made regarding the worldbuilding (ex. taming Nelf culture and forcing them to get boxed with Alliance despite the former being isolationist savages at times).
Everyone has their own bone to pick with Blizzard after 10 whole expansions.