A lot of different factors weigh into the decision. Things like a common cockpit, more similar maintenance practices, availability of delivery, better payload/range/performance, aggressive pricing by Airbus (and now, Boeing), political factors, and complacency by Boeing or Airbus if they already have most of the deals, all can play parts in an airlines decision.

As for LH, their mainline fleet is almost all Airbus, with the exception of the 733, the 735, and the 744. The Boeing planes will be phased out over time and, I imagine, LH will become an all-Airbus fleet.

Yes, the 777 would be more efficient then the A346 and A343 fleet LH uses, but LH already has the A333, A332, and A306, so there is a great deal of commonality across all five models. Adding the 777 to the mix would result in increased operating and maintenance costs in terms of spare parts storage and crew/staff training. Those costs most likely outweigh the efficiency benefits of operating the 777.

In the case of SQ, they purchased A343s and A345s because the appropriate Boeing product (the 772ER and 772LR) were not available. Once the 772ER became available, SQ shed their A343s and I expect them to place an order for 772LRs to shed their A345s.

Same with AC, which is replacing a mix of Boeing (767) and Airbus (A343/A345) models with all Boeing (772ER, 773ER, 772LR, 78x) to standardize their fleet.

Some airlines continue to fly a mix of planes. AF, for example, has ordered 773ERs instead of A346s, even though they operate the A342 and A343 as well as the 772ER. Each model has specific advantages, and the fleets are large enough to spread the costs around, so a mixed fleet works. Same with UA and their mix of Boeing and Airbus planes.