Well, did you read the article? It describes manual memory management, which is probably the most obvious reason C is lower level. It also talks about how strings are stored directly as arrays of characters as opposed to objects wrapping character arrays. Finally, C pointers are closer to how the computer operates (they're basically memory addresses) than anything in Python or Java.

What constitutes a "low level language" is a matter of perspective, however. From the perspective of writing binary instructions by hand, all three languages are "high level." ;)