Originally Posted by
kyselad
I think the chainring size is probably the main point. We can only go so small for rear cogs, meaning the lower limit for chainring teeth still leaves us with big chainrings in order to hit reasonable gear ratios. A “front cluster” would need a lot of teeth, making it very heavy and also requiring a derailleur with a huge capacity due to a large difference in absolute tooth count between the low and high end.
Width is another major factor. We would all have to ride like John Wayne.
There isn't really a lot going for the Nx1 concept, which explains why it isn't a thing.