I am surprised that you're surprised. It's not like Bitcoin was taught in school books, nor is it posted on every wall outside or every ad on the internet. It's like you're asking someone, "Why don't you know this game?" when they aren't even a gamer. If they were along the lines of developer/businessman, you can be half surprised since both have the chance of actually chancing upon Bitcoin. Besides, even the majority of the people who know about Bitcoin only know the word, they don't even know what it is.
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The OP have every right to be surprised when his two customers which is from the US seems not to be aware of bitcoin because 60% of all people in the US have heard about bitcoin while 14% own bitcoin and i dont know where you get your own statistic information.
And where did you get yours? Hearing about Bitcoin doesn't necessarily mean knowing about Bitcoin though .