Organization, population, fertile land, supporting technologies, engineering, a martial culture, a cohesive, relatively permissive religious structure, a willingness to adopt new ideas, leadership, timing, luck.

People have written whole bookshelves delving into just pieces of this question. It has no single answer.

And the legions were a fair juggernaut in Republican times too.

Unless you're asking why the legions became so politically important after the Republican era. But that's a whole different question.

How about after the Marian reforms? Do you think it was a good idea for him to get rid of the land requirements in the long, as it seems that it might have degraded the discipline in a way. But, on the other side of the sesterce, it gave them more patriotism( I think anyways). Do you think personal legions, as they might as well be called, was a good thing, or was it the root cause of the year of the five emperors?