High resolution on Amiga was interlaced and required a scan doubler to be tolerable. Reading MSDOS formatted floppies required special software. DTP and other productivity software tended to find their way to the ST first. Atari made a rock bottom price cheap laser printer. The total package price for a 1Mb ST + high res monitor was lower than anything Commodore ever offered, and far lower than anything Apple offered, but still got you an 8mhz 68000, a GUI, MIDI ports, etc.
I always try to say: the competition for the ST was the Mac and a PC, not the Amiga. Different market segment. Yes, on a low-res colour monitor many people purchased the ST as a games machine, but it wasn't great for that, really. It was a cheap productivity machine, "power without the price". More memory and more Mhz per dollar than anything else out at the time. And that's the segment Tramiel was targeting, he was going after the Mac ("computers for the masses, not for the classes."). The "Jackintosh"
For MIDI sequencing, there's no comparison. The breadth of software on the ST was far beyond anything on the Amiga and some giants of the current DAW software market like Cubase and Logic got their start there.
And honestly, while onboard sampled sound generation on the Amiga was better than its competition, that's kind of a dubious distinction when we're talking about grainy low-bitrate 28khz 8 bit audio. Not exactly CD quality. Within a few years it was outdated relative to what you could get on a commodity PC ISA sound card.