Why is it that Americans cannot tell the difference between Sikh's and Muslims?

Probably because both groups tend to be somewhat insular, so that many Americans are never really exposed to many of either. While you may see large numbers of each group in some major cities, finding a Sikh OR a Muslim in many, if not most, parts of the U.S. is highly unlikely.

Without exposure to a group or culture, people don't learn about it. When people don't have FACTS, they'll often come up with their own ideas, so perhaps it would be a good idea to do a little more outreach to your community if you are part of one group but being mistaken for the other.

As some have pointed out, some can tell the difference between Sikh's and Muslims.

Because Americans are stupid. I'm an American and I can tell the difference, but that's because I went out of my way to study Indian Dharmaic traditions (Hinduism in particular). Plus I have a couple friends who a Sikh. Really cool people.

While I think I know the difference I really don't care. Being an atheist I just look at all as a person, not place them on levels according to what myth they have fallen for.

Because of the turban, I think and being ignorant with a touch of racism

For the same reason I can't tell the difference between a protestant and a catholic. They are all human beings and I see them firstly as such.

Muslims generally keep to themselves and frown a lot. Sikhs are more jovial.

As a Christian I don't hate anyone and as I have no idea who is a Sikh and even if it is some part of being Muslim, I live in Australia, are you a jihadist is that where you live?

Why is it you're so ignorant you cannot tell the difference between a Mormon/JW and a Christian?

Because most americans are too self absorbed to think about much besides themselves. Also, americans tend to simply believe what media tells them (fox news, anyone?) instead of actually thinking.