Because the person creating the image didn't take the time to optimize the image. It's probably just a PNG or a JPEG, which is way overkill for representing a NES frame.

Other commenters have mentioned that the NES has 56 colors and uses tiles to draw the frame. If you took the same approach (maybe embedding a GIF tile in an SVG), you could cut down the size of the modern image significantly.

[–] Floey@lemm.ee 1 points 2 months ago

PNG is fine for this. A palletized PNG on max compression would be smaller than the game and totally lossless unlike this. First you gotta take this image and map/quantize it to the original palette though to destroy the JPEG artifacts, which produce colors in between ones that would have existed in the game's output.