Any reasoning behind why it only happens to Hip Hop?
From what I know and have been told by others, it's because rap fans are more rabid and consume music a lot quicker than fans of other genres. We always want the music now and get tired of it after a week. No one wants to be late on the newest rap album. So because of those realities, the record labels make a big push to get big first week sales, and aren't necessarily as concerned with everything after, especially if an album has a bad first week. They give up on singles if they don't pick up rapidly, etc.
In those non-urban genres, people discover the music later on and there's a slow burn that can build album sales over time. For example, Maroon 5's first album did terribly first week, but eventually, after about 2 years, it had sold 4 million copies. They actually won the Grammy for Best New Artist for an album 3 years after it dropped. If a rapper sold less than 50k first week, the label would throw that album in the bushes.
In rap, your true fanbase will almost always show in your first week sales. It actually happens fairly often in R&B too.
Hope that helps.