Kelly: I think you could construct a sentence, "ella está muy cansado porque del viaje," and it would be correct.
According to SpanishDict.com, "por" can also translate to "because of," as in the sentence, "Se retiró por problemas personales." - She resigned because of personal issues.
I think the use of "por" in these sentence would translate more like, as a result of (or because of)
I think it can sometimes be frustrating to newer students of Spanish (and I include myself in that group) when sentences are not translated literally.
If the sentence would have been translated as , "She is very tired as a result of the trip" it would probably have been less confusing to you. After a long time I think I understand that literal, word for word translations sometimes don't work, and that there are several ways to say many things, just as there are in English. We just have to get used to ambiguity.