I've been trying to rationalize the decision to leave those characters out, but it's not easy to do. Assuming they're not just being lazy (which may be a foolish assumption), they've obviously heard the complaints about all the superflous characters who have joined the series along the way. But the impression I get from the majority of fans is that the acceptable line is drawn after Tails and Knuckles. These characters are popular. It's Amy Rose, Big the Cat, Cream the Rabbit, Wally the Worm and Faeces the Dung Beetle that people want to hold a live cremation for.
I've tried looking at it from a storyline perspective. Knuckles lives on the Floating/Angel Island, so perhaps it's not logical for him to appear unless they set the game there again (unlikely). He'd have to glide a hell of a long way down to find the Green Hill Zone, etc. But he's a fricking awesome character - way cooler than Sonic - and they could get him involved if they wanted to. Tails could fetch him in his nice red bi-plane and return to save Sonic at a critical moment.
It just seems like they've thrown the baby out with the bath water. I'm all for ditching the post-Genesis characters but it's going to feel a little bare without the two that justifiably shared the stage for most of that series.
On the positive side, if we learn they've insisted on putting speech in the game (like in Sonic Rush), at least we'll be spared Tails' voice.