The entire effort revolves around simplicity and minimalism in circuit design.
Certainly you can get good objective performance with multi-stage circuits and
negative feedback. What we want is the sound that can be had from a single
gain stage giving both voltage and current gain operated single-ended Class A
without feedback or degeneration, and we want it with a high input impedance
and a low output impedance. And we want it with reasonably low distortion with
a simple low order character.
We can try this with Pentodes or Mosfets, but the results don't measure well,
and they don't sound as good. This is a clear-cut case where measurements
and subjective performance agree.
It is worth noting that the original efforts by Sony and Yamaha were not
minimalist – they contained many parts in multiple gain stages and used a
generous amount of feedback.
There are several reasons for the push toward minimalism. The first is simply
aesthetic – there is much to admire about an amplifier which performs well with
only one transistor. And of course there is an attractive challenge, which is
“How good can you make such an amplifier?”.