The 604e was also faster than the 750 in certain use cases. For everything else, I believe there was only a marginal speed increase.Considering it was slower than the 604e and cost more than the 604e. It makes sense Apple went with the 750. The G3 was actually a pretty cheap computer. Especially the iMac.
I don't know much about the 620. It sounds like it was meant for a specialized market that needed a lot of RAM. Way more RAM than anyone would have needed in a single computer. Which I doubt would've been utilized very well in Mac OS back then anyways. What with it's horrible multi-tasking and multi-CPU support. More like something for Unix and Linux systems. Possibly IBM's mainframes.
I’d say it was cost. 604e Macs were very expensive. Even without inflation they’d look very expensive today. The 603 was much cheaper. Makes sense to base it off the cheap chip.The 604e was also faster than the 750 in certain use cases. For everything else, I believe there was only a marginal speed increase.
What I never understood was why AIM decided to design the 'G3' PPC chip based on the 603 instead of the vastly superior 604.