Why can't this be the simple (and obvious) explanation for the dimming of KIC 8462852?
The simplest reason why your proposed explanation is not really that obvious, and thus was not considered is that the dimming of KIC 8462852 was not an isolated event. The dimming is a pattern that has been observed over years. There is no known phenomenon in interstellar space that would cause a repeated obstruction of that star, so it doesn't make any sense to consider that possibility when there are other phenomena (such as those captured in Boyajian's paper) that are more likely to be the source of the dimming.