The reason why they took prone out is because there would be too many player animations for it like moving left, right, up, down, reloading, etc. and they said they were going to have to reprogram the AI to be able to understand all of this and that it would delay the game way too much.

Also, samething applies to peeking left and right. You can't do it in single player because they would have had to reprogram the AI and tell it how to interpret that. If you guys wanted to see this, it would have taken them another 3 - 4 months to add it.