You didn't use か because you're not asking a direct question of whether there is a free table at the corner. If you were to say:
隅のテーブルがいいのですか。then it can be translated to: "Is it OK to take a table at the corner?" or "Is there a free table at the corner for us to take?"
But in your sentence, you're simply stating that you'd like to have a table in the corner, but you don't know whether there is a free one to take. が here is a Group 4 particle which means "but". The literal translation of your sentence is:
「隅のテーブルがいいのですが」"A table at the corner is good for us but, ..." As you can see, the sentence is missing a half. "Could we have a table in the corner?" is really coming from "We'd like a table in the corner but, is it possible?". It would help you to insert dots (・）after が to know that the sentence is essentially not a complete one. It's missing a half, but what you're referring to in the first half of it is understood from the context. The complete sentence can be put like this: