See this kind of thing to happen,
Is almost next to impossible,
But still for the sake of understanding let us consider that something like this might happen.
The possible reasons may be-
1) All the major bitcoin owners just stop using it and just keep it in their USB.
2) There may be some better technology than bitcoins that might show up with the passage of time.
3) And the third and the most obvious is that with the Extinction of human race bitcoins will also die.

4) Security organisations might recognize Satoshi and get to know all the loopholes of the programing of bitcoins by him.