Large piston twins just don't have the demand. 20 years ago a piper rep said they would make PA31s again if there was a market for at least 100+. They get drips and drabs of interest but the used market trumps new prices and by the time the used planes run out might be years and new technology will use a new design or the use for them is jus uneconomical. After all this is a market for working aircraft, not private use.
If you really want something like a Navajo for private use you are better off buying an old one and restoring it. It will be cheaper than a new one.