When you see a relatively condensed explanation of Roman history a lot of time between the establishment of the empire and the adoption of Christianity is ignored. There were plenty of civil and external wars and changes in politics and economy but they aren't considered that important. However, you always hear about Diocletian. And I fail to grasp why is he the most important person in 3 centuries. He made a grand reform but it didn't outlive him, didn't it? So what am I missing, why is he considered so important?
I think there are some misconceptions here which can be addressed by some generalisations, rather than cited texts.
Diocletian is important, but to say he was the most important individual for 2-3 centuries would be unwise. It depends upon the context in which you are looking at him.
In stabilising, albeit temporarily, what had been a period of instability within the Roman Empire, he was exceptionally important. Additionally, the Tetrarchy made a more manageable, arguably, division of the Roman Empire. Although the system would go on to collapse within two decades, the split between East and West would significantly change the socio-political landscape of Europe. In terms of his religious persecutions, they are of course of relative importance to Christian scholars, and to historians alike.
The final point. A condensed history of the Roman Empire would be exactly that. Condensed. It would be dependent upon the interests and agenda of the editor or historian. Someone looking at Philosophical History would maybe place greater importance upon the reigns of Hadrian, Antoninus Pius, and Marcus Aurelius. In the same way a Military Historian might place more emphasis on the reigns of Tiberius, Vespasian, Titus, Trajan, Marcus Aurelius, and Septimius Severus. It all comes down to the lens with which you wish to look at the Roman Empire.