Why is triclosan not considered an antibiotic?
I agree with @Chris that this is mostly a matter of definitions - 

Greenfacts defines biocides as: 


  According to the Biocides Directive (98/8/EC), biocidal products are those that are intended to destroy, render harmless, prevent the action of, or otherwise exert a controlling effect on any harmful organism by chemical or biological means. Examples include disinfectants, preservatives, antiseptics, pesticides, herbicides, fungicides and insecticides.


and antibiotics as: 


  Antibiotics are widely used as medication against disease-causing bacteria. However, bacteria can acquire resistance to particular antibiotics through mutation or gene transfer.


So the key difference is that antibiotics act more specific and may induce resistance. Antiseptics such as alcohol stay active, even after repeated use, because they act broadly. Now that triclosan is argued to have a specific molecular target, it may have to be re-defined from a general biocide to antibiotic. This means that its widespread use may be alarming, and I quote from your linked article (Schweizer, 2001):


  [Triclosan is used in a] multitude of health care and consumer products [and has] germicidal properties [and has] flooded the market in recent years in response to the public's fear of communicable bacteria. 


Reference
- Schweizer, FEMS Microbiol Lett (2001); 202(1): 1-7