Alrighty, I'll try to explain. Looking at the picture of the bust itself, there is a huge amount of shades of color that are way too similar to each other - the PJ Specs tool shows a grand total of 1528 different colors in there. In addition, when I zoom in onto the picture, the bust that is depicted shows a lot of artifacting, basically many different pixels of barely different shades that were randomly placed. This is not something you can do by hand unless you use non-pixel art tools, save in a bad format or spend a crazy amount of time doing, and even in the latter case you probably don't need that many shades XD
By placing each pixel by hand, it means that you control which pixel goes where and what color it is. When you use such tools like blurring, gradients and brushes with AA enabled, you don't control all the pixels because the computer automatically decides what these pixels look like instead of you. To achieve things like gradients and blurring, you have to do it using basic tools such as the pencil, bucket and line/shape tools with AA disabled. That's the gist of pixel art :)
On the other hand, if you did place each pixel yourself, then you may have saved it in the wrong format (the extension shows as PNG but I don't know if it was saved as something else beforehand). Did you save it as PNG right off the bat or did you save it in another format first? PNG and GIF are the best formats; never save as .jpg or that will destroy your pixel art's quality :(
I hope this helps! If there's anything that you don't understand, I will try to help further.

When you click Save As or Export and go to the prompt where you choose where to save it and under what filename, make sure that "png" or "gif" (the latter if your piece is animated) is selected at the bottom before you click OK. That should be enough to let the piece retain its quality :)

I'd also add that 80 is a very large number of colors for pixel art, especially of that size. Making each color do as much work as possible before reaching for a new one is a key skill in pixel art