They are taking them off eastern arc because they aren't on western arc... and they are trying to balance the channels available on west/east and will not add new channels that they cannot add for both arcs.
As for why 118.7 is in use... I forget the origins of why they have it.. but they have it...and they have far fewer international customers than they do customers in general... so someone counted beans and decided it was worth it to not have to launch another satellite for internationals and to use what they have.