It depends on what "fascinating and very compelling and well researched" pieces you're talking about, and particularly what you mean by researched. Most of those works I get asked about are virtually tertiary sources--assemblies of historians' own work, with a smattering of published primary sources here and there to fill it out. A historian goes much more deeply into the matter, and provides the basis for the work of the popular history writer/historian/journalist who may not have the training. Books on the DR Congo, for example, are usually of this sort. The result is gripping, readable, but not necessarily complete--and the authors usually address this right up front or at the back with additional reading suggestions that do operate on that more involved level. Some journalists can eventually develop that skill-set if they start to specialize in certain topics, and a few do have training as historians, but on the whole they're presenting a narrative on a topic and not making a specific argument based on new readings of primary source material.

So a historian is trained to work critically with original sources that are not contemporary to oneself, and has the knowledge needed to find where these sources are; they also have a clear grasp of the historical development of thinking on the subject in question. A journalist is arguably better at compiling a variety of historical narratives into something interesting of contemporary relevance (rightly or wrongly so) but they are less likely to possess the skills and specialized education necessary to work from the foundational material and present an original contribution or thesis.

I guess it partly depends on what we mean by "historian." Some people use the term loosely to include anyone who reads and writes about history extensively, which would include both journalists and even some people here. So amateur role.

Others see "historian" as more profession, like an attorney or doctor. Like the ABA and AMA, there is a professional organization of historians, AHA, mostly made up of professional historians, trained in graduate school, with a PhD, and often but not always affiliated with a college or university. I'm guessing this does not apply to very many people here.

Following the second definition of "historian," one difference I've found is that journalists usually aren't interested at all in acknowledging or engaging with the vast historiography already written on the topic because it's considered "boring" or interrupts the narrative, which is prioritized over analysis. I guess that's a second difference. For professional historians, narrative is secondary to analysis. For journalists, narrative is more important.

Edit: Some more thoughts. The issue is muddied because the very idea of "historian" has been so stretched and expanded to include almost anyone from a redditor who evinces a mere amateurish interest in the past all the way to professional scholars who spent years in graduate seminars, language study, archival research, and dissertation writing as a profession (not a hobby) and is part of an academic community through workshops, conferences, grants, etc. Many journalists who write history books on the side routinely call themselves as "historians" too. They are probably in the middle of that spectrum. They could be seen as "professional historians" in the sense that they make money from history books they write. But usually they did not go to graduate school, they lack the skills taught there, and are almost totally divorced from the professional world of historians as in history professors. So that's why I think it's hard to neatly separate "historians" and "journalists" because of how all-encompassing "historian" has become.

The training of an academic historian involves lots of esoteric discussion about how one ought to think about both the doing of history and history itself. What kind of narrative is history? What is the truth status of its evidence? How should one think about the big themes?

This isn't out of reach of journalists, but typically they are not as interested in that sort of thing. Why not? Because it requires a huge amount of time and study and will to actually digest. Your man on the street does not usually want to think about historiography, about epistemology, about theories of history. The journalist is typically interested primarily in telling a story about "what happened," which is part of the historian's craft, but they are not that interested in telling a story about "how should we think about the what and the happened in 'what happened'?", which is more of an academic's question.

Separately, there is the question of how to do long-range original research, which journalists are usually (but not always) disinclined to do for professional reasons. The deadlines in journalism are different than those for historians. Historians can act on scales of years or even decades of work. Journalists usually cannot. The good works of history by journalists are usually 5-10 years in the making, the same as an academic's — you just can't really get deep into a topic without that kind of experience (and that goes for any topic, not just history; there does seem to be a bare minimum of hours you have to put into anything to be truly good at it).

Now occasionally journalists do write good histories. These journalists tend to do two things differently than the ones who write bad ones. First, as noted, they take the time to really do the research. You can't just grab a bunch of books from the library and start summarizing. You'll never say anything interesting that way. Some of them find ways to take the time necessary. Second, they do dip enough into the concerns and discussions of the academics to figure out how to think less superficially about their subject matter. I've talked with a number of journalists who wrote history books over the years and while they aren't usually that interested in contributing to the kinds of discussions that only academics have, they figure out what they can take away from them to get an interesting way of thinking about the past. The really great history-by-journalist books exhibit an understanding of the academics' debates and concerns without necessarily trying to contribute to said debates.

Fundamentally historians and journalists have different relationships with their work because their training, goals, and skill sets are pretty different, even if they can superficially look similar (e.g. both produce non-fictional narratives about change over time). The good works of journalistic history are I think cousins of academic works of history, and some academic works of history can be cousins of journalistic works as well. The goals of each profession are a bit different, as are the professional constraints.

To be clear, I'm not saying that all academic works of history are great — much to the contrary, I think the publish-or-perish tenure system, mixed with academia's hierarchies, intellectual fads, and the mixed-results of peer review, produces a lot of dreck, too. The worst of journalist-written-history produces warmed over myths, the worst of academic-written-history produces impenetrable jargon, contrarianism-for-its-own-sake, and thinly-veiled antiquarianism.