Cannondale likes to make a lot of their own "proprietary" stuff. The lefty simply happened to work very well. The single leg was quite stiff and the damper being of a larger diameter allowed for the issues with smaller cartridges to be solved easier. Things like cavitation, oil sloshing, foaming, heat dispersion.... all that junk.
The cannondale frames are very very nice but they always had this oversize headtube so you could run their headshok forks. If you wanted to switch out to a different standard telescopic fork you had to get a reducer kit. More parts, more headache. The lefty was much better than the headshocks when it came out.
You wouldn't go wrong running a regular fork on a cannondale but I wouldn't go out of my way to fit a lefty on a standard frame or find another compatible frame.
The carbon lefty is just a really nice trick part too! Performance and looks!
