Tspence92: To be grammatically correct, it should read: "It is we..." 'Us" is an object pronoun and though commonly used as you say, it's still technically incorrect. Of course we say all the time, "It's me, it's him, it's her, etc." but if you turn it around and start with the predicate NOMINATIVE, you'd clearly see the error: "Me it is, him it is, her it is, etc." Colloquial English is odd for that reason. Say it one way it's "Us", say it another it's "We". Other languages such as German, e.g., maintain the nominative subject case, regardless of work order.
redbrickhouse: Because that's what's said. That doesn't make it more correct or "It is we" wrong, but living languages change with time and usage and progressive grammars reflect that change. Most speakers say e.g. "It's him" not "It's he" etc. Languages grow and adapt, its users should too. Norwegian e.g. says "Det er meg" (objective case) -- i.e., "It's me" -- while Italian says "Questo sono io." (subject case). Bottom line: Non è meglio, non è peggio, è diverso.