I’ve been over this before, but it bears repeating – print manga in the US is a lot more like anime in Japan than it is manga in Japan; it’s expensive, with a relatively small customer base. Manga in Japan only costs anywhere from 400-900yen, small enough that marketing to elementary school kids with their weekly allowances is a plausible strategy. In the US, the cheapest print manga start from $9, and can cost quite a bit more. This makes manga in the US a smaller, more exclusive market. At that price point, the proposition of just buying a $60 anime box set (there’s a pretty understandable overlap between the fanbases) starts to look a lot more appealing.
In more practical terms, manga was perpetually being walloped by anime in the US market. While the total gross of all manga tracked through bookscan in 2007 was just a hair under $109 million (all gross totals from that article unless otherwise linked), the total gross of anime DVDs in the US in that same year was over $300 million, a number surpassing not just the manga total, but the $183 million gross of the entire US comics market that same year. While more recent statistics regarding anime are hard to come by, that still speaks to a huge contrast in US sales potency for an industry where the 2013 annual gross of One Piece by itself (18,151,599*500~9 billion yen) tops the total of the top two averagers of the post-Evangelion era (Bakemonogatari and Madoka sold about 6.5 billion yen worth of disks between them).