The Nintendo can easily display 256 different 8x8 background tiles fetched from a cartridge. One could use 240 such tiles to draw e.g. a 128x120 bitmap while still having a few left over to decorate the edges, and if the cartridge fetches tile data from RAM, the content of those tiles could be updated under program control. This approach would be workable for a game like Qix, which doesn't update very much content every frame, but unworkable for Elite, where each animation frame is drawn "from scratch".
European (PAL) NES consoles only allow graphics memory to be accessed during about 20% of each frame time. That may not sound like much, but it's three times the amount of time available in North American (NTSC) consoles. This isn't enough a big enough fraction of the time to load entire bitmaps for every animation frame while achieving a decent frame rate, but it is is enough that if the system determines which tiles have meaningful picture content, it will only need a few video frames to load the data for those tiles into graphics memory. There are many additional complications, but the chief take-away is that the PAL vertical blank is long enough to make such things practical. Unfortunately, because the NTSC vertical blank is much shorter (about 7% of the frame) it's unlikely games like Elite will be adaptable to North American consoles.