Two reasons that I've heard. One, 1977 was the year that most ( all ? ) of the original band members were born. Two, it was the year that 'Star Wars' was released - listen for the sample of the TIE fighter at the start of the first track and also the song called "Dark Side / Light Side" (or something like that ) - obviously a reference to "The Force".
BTW have you found the two hidden tracks on this CD ( and I don't mean the gross-out stuff at the end ) ?