So the original Prey was about a Native American town being attacked by aliens and you playing a Native that somehow had special NA powers (that included free revivals for some reason) and you used crazy guns and portals to take them down. The NEW Prey has none of those things, except that it deals with aliens and science fiction. So why not use a new name? According to this interview, I guess Bethesda decided they had nothing better to do with the name and to just use it again.
http://www.pcgamer.com/why-prey-has...source=facebook&utm_campaign=buffer_pcgamerfb
Speaking of the original, it looks like you can't buy it anymore. I suppose that stems from the fact that the IP has changed publishers (From 2K to Bethesda) but it still stinks no one can play the original easily anymore. And it probably doesn't make sense to put the game back up for sale, since it'll just confuse the marketplace with 2 games with the EXACT same title coming out in such a short period of time.