Why are the Diadochi so lesser known throughout history and the mainstream public? Everyone has heard of Alexander the Great but why are Ptolemy I Soter or Seleucus I Nicator overlooked? Both helped start dynasties/empires that lasted for centuries. What do historians make of this?

As someone who spends much of my free time studying the Hellenistic Age, I often wonder the same thing. It is such an incredibly fascinating time period, but I can definitely see why it would be so challenging to get into.

Above all else, the challenge comes from our sources, or lack thereof Though the extant works on Alexander's life were written down no less than 300 years after his death, we arguably know more about the years 336 to 323 BC than almost any other period in ancient history. In contrast, the period running from the death of Alexander in 323 down to 281 following the death of the last Successor (Seleucus I) is unevenly documented at best, and a complete mystery at worst.
Arrian, perhaps the most "trustworthy" of the Alexander historians, also wrote a work entitled "Events After Alexander" that was only summarized in a summary by a Byzantine author. Diodorus of Sicily's universal history is our best preserved narrative account of the period of the Successors, but sharply drops in terms of surviving materials following the Battle of Ipsus in 301. Justin heavily epitomized an earlier work of a Romanized Gaul named Pompeius Trogus, who composed a "universal" history that touched upon the period of Successors.
Plutarch composed biographies on Demetrius Poliorcetes, Eumenes of Cardia, and Pyrrhus of Epirus. However, Plutarch openly admits that he is more interested in the virtues/vices of his subjects, and his work relies more on personal anecdotes and character-revealing moments than it does a strict narrative or nitty gritty details of events. Cuneiform tablets written by Babylonian priests are very unique since they provide a contemporary non-Greek account of affairs, but are extremely terse and fragmented.

To compound on this, these surviving authors relied on now-lost works by contemporaries who served in the Wars of the Diadochi, which could heavily color or bias the amount of information we know. Hieronymus of Cardia, an officer of Alexander, wrote an extremely influential history after serving with Eumenes of Cardia and the Antigonid family, so it is unsurprising that we know so much about their careers versus those like Seleucus or Ptolemy.
It also depends on the geographical relevance to our authors. Most Greek and Roman writers tended to focus on the main affairs of the Mediterranean world. Diodorus spends a large amount of time focused on his native Sicily; Hieronymus talked at length about his campaigns Asia Minor and the Levant with Eumenes and Antigonus I; Seleucus' lengthy expedition into Central Asia and his war with Chandragupta Maurya in India encompasses maybe 1 paragraph between the Roman author Appian and Justin. To make it even harder, there is a raging debate in scholarship about the chronology of events, broken down into "High" and "Low" Chronologies.

This is a major headache to sort through as a historical writer. I can also attest that writing a story on such a large cast of characters spanning 4 decades across from Greece to India, while also attempting to keep all your narrative threads together, is extremely challenging.

However, I think that Hellenistic period was seen as "less important" to the scholars of the 18th-20th centuries for a multitude of other reasons. Classical Athens and Republican Rome were sacred cows to writers and thinkers of the Enlightenment period onwards. In modern constitutional republics, we have either modeled or took direct inspiration from those democratic/republican institutions, in contrast to the "degeneracy" or "despotism" of monarchies of the Hellenistic kingdoms or the Caesars of the Roman Empire. Such views were only reinforced by the later Greek and Roman authors themselves, who especially criticized the Hellenistic monarchs on moral or political grounds (e.g. see any propaganda against Cleopatra). Prejudices regarding the malignant influences of the East on Greek civilization, and the comparative lack of interest in affairs outside of Europe and the Mediterranean did not help either. Conversely, the anticolonial movements of the 20th century portrayed those kingdoms as precursors to states like the British Raj (especially in Ptolemaic Egypt).

Thanks to a great outpouring of scholarship, there has been a huge revival in the interest in the Hellenistic period, especially with regards to the Wars of the Successors. Books like "Dividing the Spoils" by Robin Waterfield, or "Ghost on the Throne" by James Romm, along with video games like Imperator: Rome and the Total War series have brought a large amount of attention from a general audience. It is unlikely that the Diadochi will be able to command the same level attention as Alexander the Great does, but few subjects or individuals do, and we are in an upswing in terms of accessibility and general knowledge about the Hellenistic period as whole.