Monoleaf was a cost-cutting measure (more expensive for the main leaf, but cheaper than the rest of the spring pack, clamps, etc.).
Why is it more expensive now? Because it sound cool.
Mono relies on the length and degree of tapered ends for ride quality, and is very difficult to "tune" (anti-torque, ride height, rate) - a multi can be taken apart, entire leaves removed or longer ones inserted.
