That was because LK-Naxx was tuned to be easy. Also, most 'in Seunwell gear' clears were actually in Sunwell plus crafted i200 weapons gear (the Titansteel Destroyer, for example), and as melee classes were still doing 30-50% of their DPS as white damage then, the weapon mattered to an insane degree.
On top of that, Sunwell gear (and Season 4 PvP gear) wasn't nearly as far behind in ilevel as end-expansion gear tends to be now, and nor was stat-rot very strong going from 70 to 80 at the time. Unlike Legion to BfA, where each level felt like a step backwards, you got more powerful as you levelled in Northrend back then, even without better gear.