First of all, Quark was never viable on the Amiga, since it was entirely built upon Apple's Quickdraw library. When it was eventually ported to the PC, it was only possible through Quark making a QuickDraw emulation layer on Windows, and the ties to QuickDraw eventually made Quark lose foothold since it couldn't easily integrate features which weren't supported in QuickDraw.
As for Photoshop, a port to the Amiga didn't make much sense since the OCS display couldn't support the 256 colours and high resolution (640x480) necessary for useful image manipulation, and Commodore didn't support third-party graphics cards in the OS. So you can blame Commodore.
You can also blame Commodore for not doing like Atari and delivering a flicker-free high-res screen and a cheap laser printer. Atari had a sizeable share of the DTP market, especially in Germany, despite having an OS that made little children cry. 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.
