Observables are about getting and processing a "stream" of multiple values for a single subscription/request. This is different from having multiple promises, which are more conceptually multiple *independent* request/response type transactions. Broadly speaking, the observables hype seems to suggest that you can just have a single message observable and it serves the same purpose as a promise. That's not really true in all senses, but it's close enough to sound convincing as a hype statement.
But a single message observable is a much bigger hammer for the purpose of a single transaction's value. A promise is more direct for that specific purpose. It's kinda like comparing using a sledgehammer and a ball-pin hammer. They both are roughly used to drive thin pieces of metal into wood... but a sledgehammer is better for big spike nails and a ball-pin hammer is better for tiny picture hanging nails. Can you do each of those jobs with the other respective tools? Sure. But it's not a good idea.