The real reason they are doing this is because they can. Bookings are so great that they are now in an overbook situation on many cruises. I've read where Carnival had this problem and recently, a friend of mine ran into this as he prepared to leave on a Princess cruise. He was offered another cruise of longer duration and an enormous amount of shipboard credits if he took it. Unfortunately his wife works in a school system and could only travel then.
So it seems that even though many new ships are entering the market, demand is strong enough that the cruise lines can eliminate discounts and raise prices. However, they should also remember what happened to Rennaisance when they tried to play games with travel agents.