I think it's in large part that telling time is often taught poorly. Such time problems are no more inherently difficult than adding or subtracting with borrowing, or adding and subtracting measurements with conversion-- in fact those are all different forms of the same general types of operations, just with different terminology and types of "place value".
Is the fact that you think it isn't hard the only reason why you reckon it's being taught badly, Iucounu, or do you have other evidence of this? (Actually - what does it mean, anyway? If most people find X harder than Y, but a few don't, what is the difference between "X is often taught badly" and "X is occasionally taught fantastically"? Where does the norm come from?)
Personally I find it natural that children find time hard to learn, because, even though it is "just" another kind of place value, it's a different one from the one they're used to (and their first experience of the existence of bases other than 10) and it's one that is not supported by the number terminology and notation we use. If we had 10 hours in a day and 100 minutes in an hour, I venture to suggest children would find it easier to learn the basics of telling time. (No, I'm not suggesting we switch!)
