I believe the main reason is that you will have lower survivability if you self-wield infinity on a lightning sorc, versus if you were to do the same on a nova sorc.
The skill point investment for a lightning sorc is much more intensive than a nova sorc. Lightning sorcs need to max lightning, chain lightning, charged bolt, nova, and lightning mastery.
Whereas a nova sorc only needs to max nova, static field, and lightning mastery. This means a nova sorc can run with a very effective energy shield since they have enough skills leftover to max energy shield and telekinesis.
A lightning sorc can allocate a couple skills for energy shield and telekinesis, but the effective strength of their energy shield just doesn't compare.
So lightning sorcs need to worry more about maxing resistances.
The weapon and shield slots are crucial to that, because you can get a lot of resistances directly from gear in those slots(hoto + spirit), and/or get fcr in those slots so you can pick up resistances on other gear
nova sorcs self-wield infy due to having 105 fcr frame and because meditation aura makes them unkillable. vita lightning sorcs have 117 fcr and don't benefit nearly as much from meditation
a lightning sorc should always be using crescent moon + spirit monarch along with infy merc. without 35 fcr from spirit you'll end up sacrificing
a lot to reach 117 fcr. without crescent moon you're doing less damage than a summon druid