They were also already available on the website in high resolution, many of them since 2011. The difference this time is the CC0 licensing, I'm pretty sure.
A lot of people are confused about the licensing of museum images and why it's news that a major museum is releasing them CC0 when they're already public domain. I see some good thoughts here--e.g., absolutely there is a difference between 2D and 3D images (an image of a 3D work might have sufficient additional creativity involved to support a separate copyright), and yes, the Bridgeman decision is a thing. But Museums have a lot more to worry about than that. E.g., there are a lot of orphan works. More importantly, there are important people they want to be careful not to offend, whether living artists, their estates, their representatives, or potential donors. In my experience, all of those groups except artists tend to be more conservative about sharing than the staff at most museums.
This is not to say that any of that trumps the law. But Bridgeman was one weird case in one weird situation and it hasn't been tested in a museum setting. What we're talking about is not whether you can legally use the image as you like, but whether the museum will willingly show you the image. By 2011, the tide had turned and it was not going to be long before most/all major collections saw sharing as more valuable than hoarding. But in 2009 that wasn't the case.