I read somewhere that Diocletian considered the Roman Empire to be too vast to be managed by a single Emperor. But how did the early Roman Emperors managed?
And a common reason given was that Roman Emperors need to personally lead troops to deal with enemies in the Rhine, Danube and Persia. Else any local commanders who were too successful would be compelled to rebel. Why couldn't Roman Emperors delegate border defense to local commanders without fear of rebellion? Couldn't they hold their families as hostage or rotation senior commanders to different borders regularly?
From a military point of view, the strategic situation of Diocletian's Roman Empire was very much worse than that of, for example, Nero's.
Rome in the Imperial period basically had three major frontiers to worry about: the Rhine, The Danube, and the Euphrates. During much of the Principate period up to the crisis of the third century, only one of these frontiers was problematic at the time. The Parthian Empire on the Euphrates frontier was often feckless and plagued by infighting, and the Emperors were able to very adroitly prevent the various German tribes along the Rhine from ever getting sufficiently organized to constitute a serious strategic threat. Through most of this period, Rome's armies could rely upon only needing to fight one serious war at a time, and they could pretty much handle it without breaking a sweat.
A series of crises of various sorts destabilized this comfortable system. Among the first was the Antonine Plague, which wreaked absolute carnage upon the population, economy, and military of the Empire. Within a few decades, the German tribes organized themselves into the Allemanic Confederacy (Allemani means "all men" or "everybody") and started a more dedicated effort to fight the Romans for territory. Meanwhile, the Parthians were replaced in Persia by the Sasanians, who had a lot more ducks in a lot more of a row, and would go on to cause Rome serious problems, including capturing the emperor Valerian and (supposedly) making him serve as the king's footstool for mounting a horse. Pressure was increased by the arrival of various peoples from elsewhere (Huns, Goths, Vandals, Sarmatians, and others) and resources were reduced by infighting and by an economic crisis and hyperinflation.
So the Rome of Diocletion was weaker militarily, smaller in population, faced with much more dangerous strategic threats, and economically devastated from decades of more or less continuous civil war. The administrative and military challenges facing Diocletian and his immediate successors were immense, and moreover had to be faced by a polity that had just split itself in 3 parts and only been restored with great difficulty. It's not particular surprise that Diocletian thought the burden too great for one man to bear.