I'm making a campaign setting that has varying timelines. One of the larger civilizations is known as the Imperium and is reminiscent of ancient Rome. There is also a steampunk isle known as Chroborough and a series of islands similar to the Caribean in the early seventeen hundreds. How could I explain how technology hasn't spread, or adapt the different civilizations to technology that was IRL developed nearly a thousand years later?

Not much explanation is necessary in a fantasy setting, but if you still feel like it, maybe specify that the differing levels of tech have cultural significance. Maybe 1700's island doesn't use steampunk island's tech because they have different cultural values that that way of life wouldn't be compatible with. And maybe it's not even a hard rule, maybe some people in the 1700's island do think fondly of running off to steampunk land, but it's just that most people from there are fine living in their traditional fashion. 

There might be some sort of magical barrier preventing the steampunk tech and aesthetic from aspreading beyond Chroborough. Plus, simple geography might work if the Pirate Isles are on the other side of the ocean from the Imperium.

This sounds like a really cool campaign. Go for it.

What exactly is steampunk? I kinda know, but I’m not 100% sure.

Take a top hat, put some gears and a smoke stack on it, you are now steampunk. 

I'll go out on a limb and say that a Roman Empire that isn't on the leading edge of contemporary technology is the Carthaginian Empire, that is, gone.  Either they would have gotten steampunk tech to defend themselves from 1700's-tech pirates, or they would have torn themselves apart trying.

Maybe the steampunk tech is powered by magic and can't operate outside of that kingdom? Or maybe the tech is fueled using a resource only found in that kingdom? 

Thanks for all the ideas! You guys are great!

P.S. The clockworks from Mordenkainen's and Modrons from the MM can easily be reflavored to fit a steampunk campaign.

Uhhh... hello? In the Forgotten Realms canon, the island of Lantan has incredibly advanced tech, with robots, guns, etc. but everyone else is medevial. You can also explain that the more advanced societies jealously guard their secrets of tech. This leads to an dexplaination of why no one else has much of this.

Quote from OrtanFang >>

Uhhh... hello? In the Forgotten Realms canon, the island of Lantan has incredibly advanced tech, with robots, guns, etc. but everyone else is medevial. You can also explain that the more advanced societies jealously guard their secrets of tech. This leads to an dexplaination of why no one else has much of this.

 That’s cause of the god Gond, right?

Yeah, and Lantan disappearing for years until the Sundering.

Thanks.