Why is it preferable to use NPN transistors for both high-current-output devices in a push-pull amplifier?
Don't have the time to write an extensive answer, but


it's desirable that both chains are symmetric; thus, using the same components makes sense. It's easier to find to matched NPNs than if you tried to build a matched NPN+PNP pair – but both exist on the market. I hear that in the 1970's, matched NPN+NPN pairs were significantly cheaper. I don't think that's overly true these days.
For the same size, NPNs are easier to make high-current capable. In other words: once you buy a million, NPNs are cheaper than PNP able to carry the same current. Same applies if you buy very large-current transistors: both NPN and N-channel are generally easier to get than PNP / P-channel.


Generally, be wary when literature claims things without referring to why something is. Things outdate, and if you don't give reasons, you'll simply have a lot of "I don't even know if this is still true" statements in your literature.