Cap'n Proto Promises are highly analogous to promises in Javascript. Both are derived directly from the E language, which has been around for decades. It makes sense to stick with the terminology used in similar designs, so that people don't have to re-learn the same concept when switching languages.

C++'s standard library for some reason decided -- relatively recently -- to introduce the term "promise" to mean something different (what most people call a "resolver" or a "fulfiller" for a promise), which is unfortunate. It's C++ that is being inconsistent here.

There are also subtle historical differences between the meaning of "promise" and "future". Historically, futures have usually existed in multi-threaded designs rather than event-loop/callback designs; you wait() on a future, blocking the calling thread, whereas with promises you call promise.then() to register a callback to call when a promise completes. The C++ committee again seems to have gotten this confused with their definition of "future", which looks more like a traditional promise.