DreamHost is currently hosting like 700,000 domains. Imagine a huge city that everyone has some websites hosted by them. Google knows that your websites are with them by the IP which may very well has been used by some spammer or spamming sites before or even worse, you are currently sharing the IP with another spammer or slug dog rubbish-quality-seeking-money-quick kind of site. Even the IP itself has a clean history and is only used by fairly good sites, Google knows you are with DH by the IP range chart and because of the simple fact that a lot of sites on DH are not so good, it looks down upon you, with good reason.