Because one of the Bethesda developers brought up Metroid Prime as PROOF that taking something and making it first person can be a HUGE SUCCESS in an interview. I'm pretty sure it was Todd Howard.
Think about it though. Metroid was made by Nintendo. Nintendo as we all know caters to what demographic?
Children.
Yes. So it would be very safe to say that the bulk of the people who bought metroid prime were? Have we put two and two together yet?
Too young to recall the first games. Or simply aging fans of the original that are -so- desperate for anything metroid that they'll buy anything with the name emblazoned on it.
If so, you win... nothing. Except the knowledge that you're too smart to have the wool pulled over your eyes with an example so weak that only a brain damaged chimp could've thought of it.
A game like that, marketed to kids or the extremely immature will sell well no matter what. That game could've been anything but they decided to increase the street credit of the title by adding the name Metroid to it.
This in turn generated press by shitty game rags that posted nostalgic articles and headlines about the return of a "classic".
It's insulting to think they believe those who appreciate Fallout are that stupid.