Probably because it needs to avoid swaying and be earthquake-proofed. After all, the world of Japanese infrastructure design is vertically integrated so that it makes practical considerations (like making the most of limited space, or staying safe during natural disasters) a much bigger priority than anything else.
Maybe suspended monorails looked cool and steampunk-y in the turn of the 20th century as @softfurrykitty showed us, back when we didn’t care as much about accessibility, safety, or acceptance by local communities. But since then, we’ve learned that monorails are fundamentally harder and costlier to build; I don’t think it makes sense anymore in this day and age. Besides, even though they can allow for tighter turns and more flexibility in right-of-ways, it’s hard to imagine they’re cost-effective when two of the few other examples of Japanese suspended monorails (1, 2) are closing due to high maintenance costs, and manufacturers no longer sells them in the first place.
At that point, I think gondolas would be a better way to augment our future BRT lines as long as you get the business case right.