How come the existence of alternative English (UK, Canada) seem almost unheard of to a majority of Americans? Is it not taught in any way? ?

One reason could be due to the dictionaries used.  Americans use Miriam-Webster as the most reliable source whereas in Canadian bookstores one can find both the Miriam-Webster and Oxford English Dictionary (OED).  Another reason can be due to the founders of various websites such as Yahoo or the designers of word processing programmes which have Spell check with the American spelling  as one part of a menu.  

Next time you post something on Yahoo Answers and use the England spelling of a word instead of the American version you will see the word underlined as a spelling error.  The same is true for Spell Check. 

You also have to look at American History.  When the American revolution occurred one of the aftermaths was the concerted effort to eliminate the England spelling of a word.  Thus you have the American "honor", the England "honour". 

One would also have to look at the English literature curriculum in American schools.  Are the majority of authors studied American?  

Then there are the TV shows.  Regular American TV networks rarely have imported series so the American audience doesn't  have the same awareness of the language usage in other countries.  Some British TV series such as "The Office" are adapted to the US viewing audience so UK references have to be changed.  Canadian networks do import British TV series and don't change a thing so Canadians are much more aware of the language differences than their American counterparts. 

Americans are stupid.

How come the existence . . . seemS almost unheard of to a majority of Americans?