Bosses can do 2types of dmg (3including agony but that's an exception so I'll ignore it)
Direct dmg (dd) and conditions damage (cd)
Toughness is great vs dd and vitatility is great vs cd.
If out of all bosses 50/50 do dd or cd it means you either have a (close to) useless stat 50% of the time or you're running around with basically the same gear twice.
(the part below this only matters if you do not have 2sets)
So let's say (for the sake of the argument, this isn't true at all) that every 10tough or vit gives you 10% longer life time and the total amount of stats you can get equal to 100% longer life time.
If you want to up your surv by 50% you can do it by getting 50tough and 50 vit (since both are useless 50% of the time you need 50 and not 25 as you'd expect) or you can do it by getting 100vit or toughness.
This means that if you want to live twice as long you'll actually be taking 100vit+100tough or 200of either one. This in turn means that you'll have to sack your 3rd stat quite hard and you'll lose close to all dps.
On it's own this isn't important however this lower dps will make it so that fights become longer and the longer the fight the more likely you are to fck up and thus die.
Now look at it from another perspective, let's say I want to dps. Power/prec/crit all scale really well together, this means that no 3rd stat has to be saced in any way for a dps built to be "more" viable.