Well it's a bit of a catch 22, no? Either they spend a lot of time on Earth and developing your attachment to it, the people and the fictional world there OR develop the story out in space with all the alien races, colonies and mysterious unknown worlds. They can't really do both in the time allotted.
Personally, while I was always curious about ME's version of future Earth, I never lamented not going there until ME3. I mean when you get right down to it, future Earth is just modern Earth with bigger skyscrapers, flying cars and automatic sliding doors everywhere. I think Bioware quite rightly took it as read that most players were already familiar enough with the planet they currently inhabit and instead focused on the more interesting stuff. Which, let's be honest, is where the actual story is. Even then, for those that were curious, there's plenty of info in the codex to give a better understanding.