Why does the New Horizons spacecraft have so little memory (8 Gigabytes for a single buffer)? I'd have donated a few memory sticks if I could.
Two reasons why the memory is so small:
New Horizons was designed around the year 2000. Solid-state memory sizes were much smaller back then: looking at the Internet Archive, the typical CompactFlash memory card was a 64 MB card costing around $50.
The memory needs to be space-rated. That $50 card I mentioned? Outside the Earth's atmosphere, the memory will be scrambled in a matter of days due to thermal effects, solar radiation, or cosmic rays, and the card will likely be permanently destroyed over the course of a year or so.
The 8GB of solid-state memory aboard New Horizons probably cost half a million to a million dollars.
Space rated memory is much more expensive, and much less dense than commercially available RAM.
Single Event Upsets are common, even in space rated hardware, and commercial stuff would be riddled with errors due to high energy particles that the Earth's atmosphere normally protects us from.
Error detection and correction is critical in a way that commercial RAM does not care about, even as compared to high end servers.