Toughness tanking works the way that the one with the most toughness (including buffs) gets aggro.
Elementalist (especially condi) has some unavoidable toughness buffs:
This sums up to a whooping 590 (without rock barrier)! This means your tank has to really go into toughness only because of you, so it won't happen.
I don't know if it is a design philosophy to bully condi ele out of raid scenarios and if this is really intended, but if not there is an easy fix to this:
Rework weave self, the increased toughness from earth trait and rock barrier into something like "xx% reduced damage taken". For flavor reasons you could keep the 120 toughness from the weaver spec, so it doesen't look wierd on the tooltip. The 320 from weave self would even look better with something like 20%, as all the other bonuses are %based too.
This has been done on the "Rune of mercy" already, as it gives 400 toughness on tooltip but in reality gives you 25% reduced damage taken, I think the reason was the same.
Even if you think condi ele or raids are kitten, maybe upvote this so we have more future options. This would also help agression generation in general 5man content sometimes, not only raids. Of course the numbers must be balanced to not ruin PvP and WvW expirience.
An alternative (even cleaner) way would be to rework all toughness aggro mechanics to only take base values in mind, like the stat conversion traits, so all those buffs simply wont matter. But that looks more bug risky to me.
Viable != Optimal
Not viable = You only get carried, 10 players with a build as "viable as yours" can't properly do it.