Same reason McDonald's sells billions of hamburgers. No one is going to accuse McD's of being the high point of culinary expression, but folks eat that stuff anyway.
This.
Bethesda and Obsidian are big-name companies, and marketing Really Does matter in big ways. This game was very heavily marketed by Bethesda - who Knows how to market a game quite well. Marketing is the difference between us "thinking" of a product when the desire comes to mind versus have to go out and do Work to find the product. This is why so many peeps end-up in the drive-through line, its Easy.
FPS is very appealing - I played from day 1 on Doom, Quake 1, Quake 1 CTF, Quake 2, UT, etc, etc - fragged a million bad guys by now. Its fun for kids yes, but it also gets very old, very fast. The young warrior has a strong need to prove his or her skills against others, and I have felt that need on more times than I could ever recall. The single player AAA title experience is totally different, its more mature, more thought provoking and requires more skills in the brain other than superior Twitch and Anticipation skills (which FPS really does develop).
I think the comparison between FPS and RPG are bogus; its like trying to compare apples and oranges. Both are good for different reasons, and one would never go about accusing an apple of tasting like an orange or visa versa.