The lower levels are already balanced for SOs. Any tank can tank with SOs in regular content.
As for why lowbies should pick Invuln; It has good defence/resistance to smashing/lethal. It also requires no micromanagement.
It's obviously a very attractive powerset for people at low levels, considering that more invulns are played through those low levels, and taken all the way to 50, than any other tanker set.
I'd suggest you stop suggesting tweaks to powersets you don't understand.