The SNES was built to be a dedicated 2D gaming powerhouse, and in 1990 nothing came close to it. Yes, it was basically a more advanced version of what early 80s gaming systems had done - but it was
the most advanced version of what they'd done. Its direct competition was the PC Engine and the Mega Drive, and it smoked both of them.
Comparing it to arcade machines and computers isn't really fair - totally different price points and market segments. But outside of high-end workstations nothing could spit out the kind of visuals the SNES was in 1990. The Amiga 500 was, what, like $500 in 1990? At least double the price? And that could display a mere 32 9-bit colours on-screen, two background layers, no alpha, no scaling or rotation. PCs were starting to come with VGA cards standard, but you'd need to spend at least 10x as much on one to get a CPU that could do much with it. And that's not even getting into the sound hardware!
The 65816 was an unorthodox choice to be sure, but you're doing it a bit of a disservice: it's still a fully 16-bit CPU internally, and even at ~1/2 the MHz the performance delta between it and the 68k is generally overstated (because the 68k is such a cycle hog), especially for the kind of calculations 2D games tend to do.
Anyway - weaker CPU + powerful video hardware (that gets outmodded by home computers within the next two-three years) has been, with few exceptions, the model for high-end game consoles since, well, at least the SNES. It's true of the PS4/XB1, it was true of the Xbox 360 (and the PS3, sort of, the whole Cell thing is complicated), it was true of the GC...CPU is rarely the bottleneck for games, so as a rule console designers don't get too spendy there. Exceptions might be the N64 and the Xbox, although even they had CPUs that could best be described as mid-range for their release dates.