It was always a pretty important city, including being the pre-Constantinople capital of the Ottoman Empire. Wiki sez:
To elaborate, it controls the most passable invasion routes between Europe, Asia Minor, and the Middle East; it’s tangential to routes between Central Asia and Eastern Europe, and in a sense controls European land access to Africa. Like all real estate issues, the secret is location, location, location.
It’s in the middle of the natural route between Constantinople/Byzantium/Istanbul and the rest of Europe. That city has either been sending out armies to conquer parts of Europe, or defending itself against invaders from that direction, for almost 2000 years. Hadrianopolis/Adrianopole/Edirne lies at the conjunction of two river valleys flanked by mountains that give access between Constaninople and the Balkans. Basically armies going either way would be funneled through it.
It’s also been at or near the border of some major geopolitical and religious boundaries for a long time. That’s about where Islamic Asia hits Christian Europe and it’s the area where Roman and Greek civilization is bounded by the constant influx of Germans and Scandinavians moving southward and steppe nomads moving westward. You get tensions at borders. This is partly derived from the geography that everyone else has noted, and partly chance.