The true reason is thats the way most of us learned it before echo and people are set in their ways, now echo will no doubt push dps with 2 tanks if DDs aren't complete morons but honestly the fight doesnt change in the slightest with one tank or 2 just a brd/healer will replace a tank in golem phase the rest of that fight your ot will be little more than 1/3 of a dps