There are tons of reasons he didn't show it. I think it's a bit self-centered to think that it's to 'tease people who want ridley playable.'
It could be akin to the 'guess the trophy', where casual people might say 'oh man, i wonder who that is!?' because they honestly don't know, and then fans will be like 'oh my god it's ridley, how cool!' -- This is what i'm honestly guessing his intent was. The shadow was a 'tease' of ridley being a stage boss, and nothing more
It could also be to hide 'which version' of ridley it is. "Is it the prime version, the super metroid version, the other M version!? You'll have to wait and see!"
I honestly think he had no ill intent, and it's nothing more than just a 'tee hee, you know who it is!' and nothing more. Everyone here is looking at it from the perspective that ridley should be playable, which is why everyone is assuming it's a tease of his character reveal later.