EDIT: Chain Lightning actually gives chains without having cast recently, which was what I had been suggesting, and that misunderstanding also makes this feedback moot. That said, the description for chain lightning really needs an update to clarify its function. The original post follows:
Triggered Lightning blast cannot chain because the arcing power node requires you to directly cast the skill to get additional chains. This itself, isn’t necessarily an issue. Though I wasn’t playing at the time, I understand that lightning blast used to chain when triggered, and it was nerfed. However, as it is now, not being able to chain means that a huge amount of the lightning blast skill tree is simply unusable, since most of the tree is built around chains. This just simply feels bad. This also affects a significant number of effects, since triggered lightning blast seems to be practically a sub-theme for the mage class, with several different sources of triggers.
Arcing power gives an additional chain per recent cast, plus 1 maximum chain per point. Chain lightning on the other hand, does not give any source of additional chains, 1 maximum chain per point, and also drives up the mana cost. I propose that chain lightning also gives +1 additional chain per point. This would result in triggers having a more limited option to chain, at the cost of significantly driving up the mana cost. AFAIK none of the options to trigger lightning blast skip the mana cost, but if any do, they should probably be adjusted to not do that. I think this would make triggered lightning blasts much more interesting to use from any of the several sources that exist. Furthermore, if this ends up still too strong, the mana cost increase on chain lightning can be increased and used as a balancing point to bring it back down in power.