I'm not sure it's down perceived "difficulty".
The company has a pool of both Airbus and Boeing pilots, so to speak. Having flown one breed and jumpseated many times on the other to my aged brain there certainly seem to be "cultural" differences. I think it was considered easier and cheaper from the company's POV to at least initially, do a shortish type change for current/former Airbus pilots rather than both a type
and Boeing to Airbus change.
LF
Not sure about that. One of the first 380 conversions was a 747 guy.
Some of the very early trainers who converted had a mix of experience but certainly for the first few courses for the line guys and girls there was a requirement to be current or ex-Airbus. I belive in the near future even that gate will be removed and even run in the mill Boeing pilots can apply.