I believe the story goes something like this: in the google beginnings, a lot of search were in programming domain, and a lot of languages used underscores in f

I believe the story goes something like this: in the google beginnings, a lot of search were in programming domain, and a lot of languages used underscores in functions names, variables and such. Therefore, it was decided that it wouldn't be used as word delimiter, the alternative was to use dash. I'm not sure if the same applies today, but historical reasons are powerful enough that most people use dashes.