There is actually a meta-plot reasoning for not having Ackbar do this, and Rian Johnson explained it. When Holdo and Poe become contentious towards each other, we as the audience are meant to choose sides as it were, with Poe, as he's been established as "the hero". We already like Admiral Ackbar. With Holdo, we have a clear distinction between who we're supposed to choose as our "good guy" and root for. With the Dameron/Holdo dynamic, Johnson was given a chance to have a "twist", where the hero's plan failed, and the hero who we didn't like was the one who was actually right all along. that was another thing that bothered me....did they even reference that hidden planet at all before the whole getaway plan was revealed? If so I didn't catch it.