That's because the F2A didn't have to stick with using film from the early 1970s for its whole life.
The D3X is a great camera buy one! But only if you can afford to flood one. Its not likely to happen - but it could on the first dive.
The D3X does not offer anything in terms of non-sensor tech - things that actually allow you to get an image that older tech would struggle to capture (e.g. Auto-Focus) that is not already in the D300/D700/D3.
What it has is a sensor that out resolves those cameras, significantly and theoretically with better dynamic range. However, you do need to consider if the extra resolution is valuable to you needs. If it is then the D3X is the only Nikon that can give you that.
If I could afford (to flood) one I would have a D3X today. My local shop has one on the shelf - I go and visit it!
Alex