It seems to be that Boeing were perhaps skimping on humidifiers when designing the plane.
The only humidifiers I have ever known Being fit were on the flight deck and cabin crew rest areas on the 747-400. They caused there own set of problems e.g. on long flights they cause a massive ice build up on the inside of the aircraft skin which melts in descent and drips all over the back of the overhead panel and the pilots! In BA this caused a false double engine fire warning on at least one occasion.
Do any airlines have cabin humidifiers fitted?
I have flown the B747-400 and currently fly the B777. I agree the B777 seems drier but wonder whether it is because it can climb higher earlier and therefore spends longer at higher altitude than other aircraft.
To BA's credit I do know that they have spent a great deal of money and time researching the reported problems and have made several modifications to airflow patterns to try and improve the situation.