Because they're the ones most people drool over. Personally, I've always thought Imagine was kind of annoying, I don't know why. Whereas I used to think Yesterday was a bit hokey, but having listened to it again more recently I thought it was really beautiful.
And Octopus' Garden was just added for comedic value, but seems to have a following all its own. Maybe I should've included a George Harrisson one as well (Here Comes The Sun, While My Guitar Gently Weeps maybe?). Meh, too late now.
I prefer "Imagine" because of its significnce. I mean, what if everyone had no religion... and...well, you know how the song goes. Things would be a lot more peaceful if we had nothing to kill or die for.