Why is the upper respiratory tract so vulnerable to infections?
Mostly because it is in almost constant contact with aerosols from the outside world (with regard, of course, to a thin layer of mucus), therefore any pathogens they might contain. 

You also have to remember what separates an airborne pathogen from any other pathogens- infection location. A pathogen is airborne when it infects and irritates the throat or nasal passage enough to make the organism's normal deterrence functions kick in. In most cases this would be coughing or sneezing, which produce contaminated aerosols for other organisms to inhale through their own upper respiratory tract.

I hope this answers your question!