The Seleucid state was for the longest time the largest of the successor kingdoms, controlling the heart of the former Achaemenid Empire and many of the world's most affluent regions at that time. Despite that its memory in western Europe, when indeed it is remembered at all, is frequently one of precipitous, inevitable decline, and it's often regarded as nothing more than a curiousity. I'm glad that you've asked this question because it's an opportunity to talk about the Empire itself whilst also examining its memory in historiography, the traces of which lie beneath the question you've asked.

The Seleucid state is a fascinating creation, but many facets of the Empire pointed at to indicate the seeds of its decline are not unique to the Seleucids. The Empire was not especially sprawling compared to the Achaemenid Empire, and the Seleucids inherited a robust network of bureaucracy and infrastructure designed to govern and maintain order across its diverse provinces. Aramaic bureaucracy was maintained but gave way to a Greek speaking one over time, even in the farthest reaches of the Empire such as the province of Bactria. Nor would one say that it had especially venomous or frought dynastic politics compared to the Achaemenids who came before, who had fought several internal civil wars across their two centuries of existence, or indeed the Neo-Assyrian Empire which also saw civil wars between members of the royal family for possession of the throne.

There is also a persistent idea that Greeks and Macedonians were especially unwelcome to Near Eastern peoples for various reasons, leading to a deep rejection of Greek culture and rulership. Whilst the Achaemenids had pedigree in the Near East, with Persia functioning as the successor to the Kingdom of Elam, they had been just as foreign to most of their conquered subjects as the Greeks had been, happy to absorb those traditions that would lead to stable rulership but otherwise clearly marked as foreign and at times privileged above other peoples within the Empire. Nor am I arguing simply on the basis of likelihoods- it's quite evident from material culture alone that, far from rejecting Seleucid interpretations of Greek culture, the Parthians absorbed many of them wholesale and this formed a strand in Near Eastern material culture.

If anything we also need to think about why this is a monumental decline, when the Achaemenid supremacy over Middle Eurasia was broken across a single decade by a former client state that was having to maintain a logistics chain across the Hellespont and, eventually, the whole of Anatolia. If anything the overnight reversal of uncontested might into total submission to a hostile power is more shocking than a gradual loss of territory, prestige, and military power.

However, there were issues unique to the Seleucids that had not been the case for the Achaemenids, and this definitely relates to why the Seleucids lost their power bit by bit instead of, as with the Achaemenids and Neo-Assyrian Empires, being utterly vanquished at what seemed to be the height of their powers.

For one thing, the Seleucids were never actually in control of their homelands for the entirety of the Empire's existence. This resulted in the mass import of Hellenic colonists across the Empire to form citizen populations, often at times for entirely new cities. This afforded the Seleucids quite a lot of power; not only did they often have garrisons in important cities, as the Achaemenids had done, there was often a theoretically loyal population of fellow Greeks who would often dominate the local economy. This settlement was especially dense in the area we call Syria, which is why that was the element of the Seleucid state which survived the longest, and this was eventually the ideological 'homeland' of the dynasty and its Greek populations rather than Macedonia and Hellas. However, this did mean that there was the possibility for socio-economic tension in these cities, and the roving nature of the Seleucid monarch meant that these individual cities had a lot more power than it might otherwise seem. The frequency of campaigns and the fact that the King was constantly moving from place to place meant that cities had to be relied upon for logistics and provender, and these needs were not always met (though the consequences for such defiance were usually severe).

This is itself symptomatic of perhaps the biggest difference between the Achaemenids and Seleucids; the Seleucids had peers. The Achaemenids never possessed a neighbouring power that could be said to contest their might or rival their amibitions, and when an exception came in the form of Phillip II and Alexander III of Macedon the result was the wholesale replacement of the Achaemenid monarch on top of the intact structures of the Empire, with Alexander even marrying into the dynasty. By contrast the Seleucids faced a number of other successor states that could not simply be overwhelmed or overawed. Military threats to the Seleucid border, particularly in the west, were organised. The fact that there were competing powers in the Mediterranean also gave scope for marginal territories and cities to comparison shop, flitting between allegiances to get the best deals on offer. Dealing with this whilst also dealing with a geographically sprawling Empire was not easy. The Achaemenids had themselves struggled with some of the Empire's marginal areas, particularly in the Aegean and in Egypt, the latter of which successfully revolted against the Achaemenids twice. For the Seleucids they began to struggle with Central Asia, precisely because this area was not a source of constant threats.

Attempts to reassert authority in Central Asia, such as Antiochus III's campaign into Bactria, were lengthy and involved the King's person, which took him away from matters involving the other hostile successor states and cost considerable resources. This was not dissimilar to Xerxes' expedition to Greece, where he eventually had to leave and the attempt to secure a Greek satrapy collapsed not long afterwards, except that Xerxes had been attempting to push the limits of the Empire's authority and Antiochus III had been attempting to reassert his Empire's existing integrity.

The Seleucids were also existing in a larger world. When Alexander had launched his expedition against the Achaemenids they had been content to maintain an Indus frontier, with no evidence really existing that serious attempts were made to conquer further into India. Within a few decades the Mauryan Empire would come into existence, uniting most of the subcontinent and potent enough that the Seleucids, at arguably the height of their powers, were willing to part with eastern territories rather than waste time and energy in protracted warfare. Considering the stability of the Seleucid/Mauryan frontier afterwards this choice seems to have been vindicated.

We also have the elephant in the room that is the Roman Empire, growing from a regional Italian power to one of the Mediterranean heavyweights to the uncontested owner of the Western Mediterranean. The Roman Empire's decision to expand its sphere of interest to include Greece and the Aegean, and then the entire Eastern Mediterranean, denied any possibility of a belated Seleucid conquest of Macedonia or Greece, along with the possibility of a final overthrow of Ptolemaic Egypt. These gave a kind of half life to the Empire's existence, unable to overcome obstacles to its power in the west even after great efforts, with many of its former client states and breakway territories turning to Rome as the new game in town, and with its pretensions to military supremacy and independent foreign policy broken at the hands of Rome. Dynastic squabbles concerned ever smaller versions of the Empire as former satrapies asserted independence or went to Rome. Likewise its neglect in Central Asia had allowed the Parthians to go from a minor kingdom to an expanding power that had pushed the Seleucids out of the Iranian Plateau entirely. The last serious attempts the Seleucid dynasty made to assert themselves were the final campaigns aimed to defend Mesopotamia from the Parthians, which ultimately failed, dooming the remnants of the dynasty to fight over increasingly splintered parcels of the Empire's heartland in Syria.

A glib summary might point to the Seleucids as the victims of socio-economic change, as new centralised states emerged where previously the Near East could likely have written off the locals as barbarians barely capable of living in stone houses, or indeed that they were victims of the beast that Alexander unleashed; the idea that any scrappy but organised smaller state could take on the Mighty Empire and win it all. But realistically, given Seleucus' origins as a talented but also lucky grandee of Alexander's military circle, who had overcome any number of similar men to even create his Empire, they did a pretty good job at being a functional Near Eastern Empire. The fact that they were not able to eliminate their close rivals and had similar origins and motivations to them is probably the biggest specific structural flaw built into the Seleucids from the beginning; it's hard to make yourself seem quite as scary as the Achaemenid monarch, uncontested Emperor from Sardis to Samarkhand, when you are contested and when you know the King of Egypt, King of Macedon, King of Pergamon among many others have exactly the same origin you do; a Macedonian warlord with ambition and endurance. But the fact that they are considered so symptomatic of decline, so weak and ineffectual, is primarily a result of hostile Roman historiography and 19th-20th century imperial ideology that characterised them as effete, sensuous, and oriental, especially as a dynasty that mixed with Asian peoples in marriage and government. The study of the Seleucids has moved on from that idea, it's just rather taking a while to catch on.