Well first, let's define "pop history." A lot of what follows is necessarily going to be determined by how one defines the term. For instance, we certainly wouldn't want it confused with public history! For the purposes of this question, I'm going to define "pop history" as history written primarily for a non-academic audience written by authors who are also, primarily, not academics (or at least, not historians).
Some of it is credentialism; it happens in every discipline, and history is by no means an exception. Such is academia. However, we should emphasize that this is only some of it. The reality is historians often have good reason to be skeptical of "pop history." History is not simply gathering facts à la an investigative reporter sent back in time. You can't simply gather a bunch of medieval documents and start writing; in order to properly interpret and contextualize primary sources, you need to be trained in historical theory and methodology, as well as the (usually quite extensive) secondary literature on your field of study. By and large, "pop historians" do not have this kind of training. Therefore, many pop histories contain cherrypicking, anachronisms, and misleading or false interpretations.
Additionally, an important part of academic history is peer review. The academic historian is writing to an audience that knows just as much as she does about the field, and her career and reputation are at stake in being able to marshal convincing arguments and rock-solid research. In practice there are points where the process fails -- cf. Arming America's ignominious fall from grace as winner of the Bancroft Prize to the most well-known case of fabrication in the American historical community -- but overall it works well enough. "Pop history" is aimed at an audience that is often unfamiliar with the subject material and unaware of how to evaluate sources. Pop history books can sell by pandering to a political line or sensationalist narratives without any institutional or scholarly checks on the author.
All of this means that many pop historians are not up to the task of educating the public about history. Worse, because they much more widely read than academic histories, they can actually damage the public's understanding of history. Kate Larson, for instance, wrote this in a review of Killing Lincoln by Bill O'Reilly and Martin Dugard (which was on the NYT Best Sellers List and got a made-for-TV film):
Much of Killing Lincoln is a regurgitation of well-worn myths and misinterpretations of the assassination story. This would be inconsequential if the book had not achieved such wide readership but, instead, it complicates decades of effort by noted historians and researchers who have painstakingly documented the plot to kill Lincoln.^1
None of this is meant to imply all pop history is bad or poorly received by academic historians. Ron Chernow, for instance, has no degree in history and is a journalist by training, but his Hamilton biography is generally well-received as far as I understand. However, it's important to understand that not every pop historian is a Chernow, and that historians' frustrations with pop history are often rooted in real trends.