Bicycle Mechanics - chain cover

It seems that for commuting in winter climates one's chain is always battling water and dirt. Why aren't chain covers more popular?
Is it possible to get one for a derailleur drivetrain? I haven't seen much, even for single-speed/internal gearing system.

Reasons for not having a a (fully enclosed) chain cover:

- adds extra weight
- increases wind resistance
- costs more than replacing a worn "standard" chain
- can not fully enclose a derailleur chain + drive train

Reasons for having an enclosed chain cover:

- extends chain's lifetime
- allows for lubing chain excessively with heavy oil, while keeping rider's ankles (socks) clean
- less frequent chain cleaning and lubing

One can install a chain guard that covers only the top and front part of a chain on a derailleur commuter bike. It will protect fairly well against dirt and water projected by the front wheel.

A full chain guard, the kind that completely encloses the chain is a pain when you need to remove the wheel to repair a flat. And you are more prone to flats in wet weather. There is also the issue of trying to make one that will work with deraileur drivetrain.

They're popular in every country besides the United States it seems

Full chaincases are a singlespeed/IGH afair only but chainguards can be used in a single ring up front and multispeed derailleur in the back.