Not true at all. It's just the case that many academic historians are neither good nor engaging writers.

John Heilbron's Galileo is both an engaging narrative and historiographically important. He argues that Galileo was an acerbic character who was raised in a very particular element of Florentine society. Heilbron goes on to argue that Galileo's education and early acquaintances, especially with literary critics, played a significant role in his Dialogue on the Two World Systems and his subsequent condemnation.

Similarly, Alan Brandt's The Cigarette Century is an engaging analysis of how big tobacco managed to gain power in the United States, conceal evidence, and then gradually fall from power. The same is true of Robert Proctor's The Golden Holocaust, which documents the ways in which big tobacco became a pernicious influence. A.W. Crosby's work, such as America's Forgotten Epidemic is generally engaging as well.

So no, there is not an exclusivity between engaging narratives and historical complexity. All of the works I've mentioned are well-written and make important historiographical contributions. Most historians value ideas and argument over writing skill, which is good for the intellectual side of the discipline, but deleterious in terms of public outreach.

This is actually a more complicated question than it appears at first glance. Most academics say that they love engaging history and try to write entertaining stuff, but the reality is, what the field of academic history really values---and what professors have to do to get tenure---is produce original primary source research. This original research must be published by an academic press with an intended audience of other scholars in the field. It must study something that has never been studied before or provide a significantly new interpretation. Their writing is not intended to entertain, but to break new ground using previously un-mined primary sources. It can be boring; it can be well written; it can be narrative. What matters is that it is original primary source research. Most academic historians genuinely love this part of their job: going to the archives for weeks, maybe months. Slaving away in an office by yourself to put together hundreds or thousands of primary source documents into an original argument.

They look down on popular history because it rarely is "good" history by those standards. "Less boring" books are often written as narratives, rather than arguments supported with footnoted evidence. Narrative histories often leave out the nuance and complicated details that academic history requires. They sometimes aren't based on new primary sources at all, but secondary sources. They aren't always about groundbreaking archival research. They are often about topics that everyone else writes about: wars, great people, moments of great upheaval, adventures, explorers, weapons, etc. For the most part, they are written about the United States and Europe. Some of the more engaging popular histories don't cite their sources at all, or just have a bibliography at the end of the chapter.

I think the disdain stems from differences between the intended audiences of their work (other scholars vs. a popular audience), the forms of writing they have to produce to appease those audiences (argument driven monograph vs. narrative driven story), their social networks (other academics whom they met in the archive or at conferences vs. other writers or readers), their incentives for writing (get tenure vs. money and good writing), and their training (learning to analyze and write monographs vs. learning to write stories, field pieces, etc.).

But these are all generalities, right? You can have really engaging, ground-breaking monographs, and you can have really boring popular histories. But for the most part in practice, they ARE mutually exclusive because the worlds they are writing for and producing for are so different.

Personally, I think they are both fine ways of writing if done right, but the more time I spend in academia, the more I dislike popular histories. Like....why are there SO many books about the Civil War? Why? There are so many other conflicts. Why don't more people want to read about the Paraguayan War or the Haitian Revolution? Or why don't normal people in the modern world like reading about other normal people in other centuries doing normal things? I think that's really cool, but other people think it is boring. Also, the more good academic books I read, the more clearly I can see that sloppiness that less well trained writers use. I see places where there should be footnotes saying where they got their info. I can see when stories have been washed of their nuance more easily. I can see places where they should add archival material, but they don't. All of these mildly annoy me now. Am I becoming a boring historian too??

I would contend boring is dictated by the audience. One of the papers I wrote and presented would be considered a boring topic by many, I assume. Although, for me what makes the topic so interesting is how little the topic has been explored. Traditionally exciting topics have been explored by many people many times, so there is already a lot to learn but not a lot to discover.