Actually the one at Venetian grosses about $25m a year, IIRC. Far more than any of the other Grand Luxes (or Cheesecakes), in part due to its 24 hour operation.
And the real reason they're using GL as their growth concept is because many of the Cheesecake Factory leases have radius restrictions, so they can't put another one within X miles. That was why they created Grand Lux in the first place for the Venetian - there's already a Cheesecake at the Forum Shops and the Venetian was too close.

Obviously with Las Vegas Sands as the landlord for both, they're perfectly ok with one at Venetian and one at Palazzo...