Because nobody will buy them.
^This.
Where was everyone when F-Zero GX came out? And that game is from an established series!
Completely new games are very risky to make nowadays and the only ones to blame are the consumers.
Nintendo is probably the company who has released more IP's in the last 10 years... sadly, people only buy the main franchises...
just to mention a few:
-Trace Memory
-Drill Dozer
-Geist
-Chibi-Robo
-Hotel Dusk
-Disaster
-Endless Ocean
-Captain Rainbow
-Zengeki something
-Flingsmash
-Steel Diver
-The three "Operation Rainfall" RPG's
-Pushmo
-Dillon
-Wonderful 101
and I'm sure I'm missing a lot.
^This too.
Most people don't want completely new games, they just think they do.