Because anything over 6MP won't do anything for 90% of the photo printing you will be doing. Unless you are doing crazy crops like Gripster said the extra MP will just end up taking up more valuebale space in your memory and hard drive. More MP only really comes in handy for huge prints (poster).
Funny you mention Upgrading because the d80 replaced the d70 so really an upgrade would be more along the lines of the d200, and from what I have read the d70 was consructed with much stronger plastic then the d80. It's your money but it would have been better spent on a new lens as the body will do nothing for your photos it will only make taking the pics a bit more convenient.
I have been shooting with the d50 for a while now and I see no reason to upgrade at this point. I could have a d200 tomorrow if I wanted but I would rather keep that money in my pocket for now, you see bodies and lenses will be replaced every 1-2 years and you could spend a fortune trying to keep up with the companies latest and greatest but none of that will matter for producing wonderful photography. A photographer should be measured their experience not their equipment. It's funny people always ask me what I use but it really does not matter, 99% of photography is in your head.
Here is a link to Bruce Percy's portfolio
http://www.thelightandtheland.com/pa...g/eigg_02.html possibly some of the best photography in the world. He has some interesting things to say about technical gear and how it does not play into his images. Remember the word “image” comes from “imagination” and has nothing to do with noise levels and lens sharpness.