The usual: curiosity; deriving security lessons; research practice; prolonged & severe irritation with the incompetence of other Satoshi hunters; the outside chance of undying renown & deathless fame.

EDIT: Seth Robert furnishes a handy example of the crappy reasoning & rampant confirmation bias in such discussions: http://blog.sethroberts.net/2014/03/11/nick-szabo-is-satoshi-nakamoto-the-inventor-of-bitcoin/