Because Linux is painfully slow on PowerPC. Always has been.
Agree that some things is getting more bloated in Linux (don't get me started with systemd) but if you get older and optimized versions of Linux like
@wicknix made available.
I use Void really minimalist installation in all of my machines (even x86_64 or Raspberry Pi 4). The Desktop environment I'm still trying to get a nice balance between features and performance, because for more eye candy that KDE or Gnome it's (and I don't like that much) it's very resource hungry. I would love to have an iBook G3 Clamshell for having OS9 and other things, but in every machine that I have (G4's and G5) it's maxed of RAM and SSD that helps a lot, but I keep my expectations of screaming performance controlled, because having access to "modern software" on this old machines has it's price.
I will respond by asking what ISN'T slow? Especially compared to MorphOS?
I agree that MorphOS it's really fast. It has some good points like:
Good microkernel
very optimized programing languages and libraries targeting our machines.
But the points that I don't like:
They don't have any roadmap about their development,
there are many closed source software meaning that they would do whatever they like with the OS and users can't complain.
Very limited hardware support.
The OS won't use/recognize more than 1.5GB of RAM.
Don't have a great library of softwares, meaning that you have a screaming fast machine with very good optimization, but you don't know if they will move to other architecture because it's getting harder and harder to get compatible machines for any amount reasonable.
With the RAM limitation shared across the whole OS the software can't grow in complexity (No I don't like any modern browser that uses +1GB of RAM for no good reason). Their price of 80 euro it's not very friendly but what I don't like it's very clear that even paying for the software you don't get support for an OS with almost none documentation and more niche community than Linux ever was.