Why is a model quadcopter 'better' than a helicopter? Why doesn't this scale up to the full-size versions? EDIT
  
  Soon after I posted this, it was pointed out that a similar question
  exists. That other question answers most of mine. I shall leave this here as a stub. That way it provides a pointer to the other question and, importantly, my title contains all the keywords 'model', 'quadcopter','helicopter' and 'scale up' whereas the other question's title (Why haven't quadcopters been scaled up yet?) is less comprehensive and therefore less searchable.


Model helicopters were on the market for many, many years before quadcopters.

Presumably most of the engineering bugs had been removed by then as well as control issues.

Suddenly along comes the new kid on the block and it's all change.

Related questions


Why are model quadcopters so popular in relation to model helicopters?
How come the same reasons don't apply to full-size helicopters and quadcopters? Why don't we see search and rescue quadcopters and personnel-carrying quadcopters?
Cheap MEMS gyroscopes have been on the market only since maybe 10 years. Before you needed more bulky and expensive gyros. Also, cheap and powerful micro controllers which can process the sensor data quickly enough and can control the speeds of the four motors to stabilize the quadcopter have only been available recently. The third recent innovation was more gradual, but also the storage density of batteries was not available maybe 20 years ago. With modern LiPo batteries you can have flying times of several minutes where NiCd batteries only gave you a fraction of that time for the same mass.

For your second question: The reason is similar to why birds have not been scaled up to carry humans, like horses do. This answer should give a good explanation.