Only because Kharn appealed to even greater killing.
If Angron killed all his legionaires he would be kinda stuck.
If Angron killed all his legionaires he would never be able to have revenge against the emprah.
But if Angron accepted to lead the World Eaters, then together they could kill forever.
Even berzekers are reasonable enough to know to stop fighting when the weather starts getting cold enough to freeze them in their power armor, while Kharn was the one going KILL! MAIM! BURN! back in Skalathrax.
More in particular, Kharn didn't become the most famous World Eater for being a calm reasonable guy, quite in the contrary.
Despite his CRAZED nature, Kharn will still offer his services to brave Chaos warlords. However many are reluctant to call upon him, for his allies are often as at much risk as his enemies.[
That's right, even the bravest chaos lords are afraid of Kharn the crazed teamkiller.
He may have a moment of lucidity or two once every 1000 years, but the rest of the time he's been rightfully building a reputation as one of Khorne's more favored servants.