For professional PC users one additional thing was important. The display refresh rate.
In those days you put up with a lot of crap if you had to sit in front of the computer for the whole day. That’s why black screens with green or amber text was popular. The monitors were cheaper, but also you did not notice the screen flicker so much. The popular Hercules card had a higher Hz rate than the usual Color graphics and was considered for professionals. Despite the simple look. A while it was accepted to have lower rates, but in the beginning of the 90th the display refresh rate was a thing.
The Amiga had great graphics but especially in Europe with the 50 Hz PAL system they flickered too much. Even at that time 50 Hz were seen as unergonomic. The US and the Atari ST with 60 Hz were a bit better but in general 70 Hz was seen as necessary.
The Atari ST mono had 70 Hz but a small monitor. Still quite successful in professional places. If you just got away with it at the end of the 80th, in the early 90th the display refresh rate had to be 70 Hz.
At that point the graphic of the Atari ST and Amiga was not that impressive anymore compared to the PC (the ET4000 graphics chip was out) and if the ergonomics don’t fit either then it was hard to argue that they were a good fit for professional use where you use the computer longer during the day, like text processing etc.
Later Amigas supported VGA timings. The Amiga 3000 through de-interlacing "scandoubling" "flicker-fixer" hardware, which had been an add-on for Amiga 2000. ECS and AGA chipsets had "Productivity Mode" but only desktop programs used them, and ECS only with up to four colours on screen at once.
I used an Amiga 1200 with a VGA monitor for a few years. Because video modes were programmable, I traded a little refresh rate for higher screen resolution: 704×520 @ 50 Hz or thereabouts.
The Amiga had great graphics but especially in Europe with the 50 Hz PAL system they flickered too much. Even at that time 50 Hz were seen as unergonomic. The US and the Atari ST with 60 Hz were a bit better but in general 70 Hz was seen as necessary.
The Atari ST mono had 70 Hz but a small monitor. Still quite successful in professional places. If you just got away with it at the end of the 80th, in the early 90th the display refresh rate had to be 70 Hz.
At that point the graphic of the Atari ST and Amiga was not that impressive anymore compared to the PC (the ET4000 graphics chip was out) and if the ergonomics don’t fit either then it was hard to argue that they were a good fit for professional use where you use the computer longer during the day, like text processing etc.