If I was going to make a DTP package in 1987, I would also have chosen the Atari over the Amiga, simply because the price for an equivalent Amiga package would have been twice that of the Atari, given the extra expense of the flicker fixer, hard drive and laser printer.
Yes. Times were different, people who had never used a computer before would buy one to run one piece of software.
The ST hardware was inferior in many ways but that mainly affected games. The built in midi, hard drive port & progressive high resolution video gave it a real boost in certain markets.
I was playing games and wasn't doing DTP or image manipulation, so I had an Amiga connected to an old TV set.