It costs money to keep running a gimmick
Seriously though, I think they expanded at such a rate that there hasn`t been enough time to do the paintjobs. I somehow recall hearing an interview with BK where he said something like that, and that he didn`t find himself worthy of having his face on the tail of a 737, as all the "real Norwegians" we either pioneers and/or heroes, and that none of them are with us anymore.
The definition of "flag carrier" is somewhat different to what you are refrring to, though I get what you are trying to say
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Funny how there are no "real Swedes", "real Danes" og "real Pollacks" on the tails either, but hey, this takes time, research and money