I want to open by saying that I think Philip I of France is a super interesting king, and I totally agree that we should talk about him more, but I would also say that quantifying exactly why a historical figure is ‘neglected’ is a tricky thing – there’s even room to debate whether a historical figure is genuinely ignored in scholarship. I don’t know about you, but I’m definitely not an expert in French historiography and so it’s totally possible that they spend a lot more time talking about him than English language historians do. I suppose my big caveat here is that I can’t provide you with a definite answer as to why Philip I doesn’t seem to be as central a figure to political history as you might expect, but I can suggest some reasons.

One of the greatest factors is probably that while those big three events you listed in your post happened during Philip’s lifetime, he wasn’t particularly involved with any of them. Take, for example, the Conquest of England by William of Normandy. William’s life tends to be covered in two periods: his youth as an illegitimate child raised to the status of Duke and his later conquest of England. Obviously a lot more happened during his life, and a detailed scholarly biography will cover all that, but if you’re writing a survey history of Medieval Europe, those are the two bits of William’s life you’re likely going to cover. William’s youth and rise to power all happened during the reign of Philip’s father Henry I, and Henry plays a huge role in the story of William’s rise to power. By the time Philip took the throne in 1059/60 that was pretty much over, and the conquest hadn’t yet begun. The Conquest narrative often neglects the role of the French king to focus primarily on the Anglo-Norman side of things (which is understandable) and so Philip doesn’t really play a role in that history, meaning he gets side-lined if not left out entirely.

The Investiture Contest is an even more extreme case. It’s a really complex and important piece of history, but one that is often framed nearly entirely as a conflict between the papacy and the German Emperor. While later Papal-German conflicts pulled in multiple rulers (see for example the complexity around Philip II, Otto IV, John, and Frederick II all getting pulled into the same weird meta-conflict thing), at this stage it’s not unreasonable to see the Investiture Contest as primarily between the papacy and Emperor Henry IV. When Philip is mentioned it’s usually for how elegantly he managed to extricate himself from conflict with the papacy (over this issue anyway, in the grand scheme of things he wasn’t brilliant at not angering popes), which is impressive but also usually succeeds in extricating him from the narrative as well.

The First Crusade is usually the area where he probably deserves the most mention, but he often just gets pushed out by the sheer number of other personalities involved. For crusade leaders who participated in the First Crusade you have to cover: Robert of Normandy, Hugh of Vermandois (Philip’s brother, as mentioned elsewhere), Raymond of Toulouse, Bohemond of Taranto, Godfrey of Bouillon, Baldwin of Flanders, Tancred (later prince of Galilee), Alexius I Emperor of Byzantium, Pope Urban II, Adhemar of le Puy, and at least a half a dozen other people. That’s a big cast of important figures, and while you can get away with focusing on a few of them and leaving others out, you can see how the King of France, an excommunicated ruler who didn’t participate, kind of has a habit of being left out of the narrative. It’s not that he wasn’t involved, the pope living in exile in his kingdom (albeit not necessarily in areas he controlled directly) while preaching Crusade (and condemning Philip, interestingly enough) is an interesting and important part of the early history of the First Crusade. It’s just that there’s so much else going on that nobody has time for tertiary figures.

When Philip does come in to standard historical narratives it’s often in the context of the Capetian monarchs. If you read Christopher Brooke or RHC Davis’ surveys of the period (classic texts that you’re likely to encounter in any entry level college course), they describe Philip as the beginning of a series of Capetian kings that began to turn the French monarch from something of a backwater (perhaps an unfair characterisation) into a central authority figure. Philip is often portrayed as the start while his son Louis VI and grandson Louis VII greatly expanded that authority and his namesake grandson Philip II ‘Augustus’ brings the French monarchy into a new era by pushing the English out of France (for the time being anyway). In these narratives Philip II is sort of the ‘real protagonist’ if you will, and Philip I’s role is the force that laid the groundwork for those later achievements. He’s an important figure, but he basically gets stuck sharing the stage. It doesn’t help that Louis VI and Louis VII had Abbot Suger – one of the most important clerical figures of the period – as close friend and advisor which really helped boost their narrative and cement their centrality to the overall story. He wrote a glowing biography of Louis VI, and while we obviously don’t buy into all of his propaganda having a big source like that does help when it comes to actually writing history.