Why don't cyclists wear all-encompassing motorcycle-style helmets? It is possible to get a helmet that covers your entire head and face, and in theory would protect you against more kinds of falls. In particular, Why don't bicyclists wear them, and why do motorcyclists wear them? 

I'm not asking whether helmets are a good idea in general. (Please try to answer this with complete answers, rather than the single-point style of the currently running helmet thread.)
Some bicyclists do wear full-face helmets; in particular, downhill mountain bikers and BMXers will frequently wear full-face helmets, as the chance of a crash causing you to land on your face is greater.

The problem with full-face helmets is that they're hotter; your head is great for getting rid of excess heat, and the full-face helmet helps trap more of that in. Motorcyclists aren't pedaling furiously, and don't need to lose as much heat; heat management is essential for bicycling.

Furthermore, motorcycle accidents tend to happen at higher speeds. On a bike, you're pretty unlikely to wipe out at 60 MPH, while on a motorcycle, it's all too possible. Between the lower risk on a bike, the heat issues, and the extra cost of full-face helmets, for most people it's not worth it to wear the full face helmet on their bicycle.

Any safety feature is a trade-off, between the costs of implementing the feature and the harm or damage that it prevents. When the cost of implementing the feature exceed the expected value of the feature in harm reduction (taking into account the likelihood of the particular form of accident happening, and how severe the damage is), then it doesn't make sense to use that feature. People all assign different values to certain types of harm or damage, and also have different risks based on how they ride and what their environment is like. For some people, wearing a full-face helmet may be worth the extra expense and heat, but most people don't find it's worth that trade-off.

Sadly, for many of these decisions, we don't have enough information; there isn't very much conclusive real-world data on the efficacy of bike helmets, let alone different types of bike helmets.