Unfortunately it takes a lot of money to keep it up and running (not to mention hosting it online) and I think the creators were running into a lot of problems where people were using it but not contributing any money toward the project, or people who got a copy and illegally shared it with a ton of other people. I know the professor who runs it once mentioned that at one point he was getting several emails a week from people gushing and thanking him for making the program... from people who never purchased it and didn't even realize it was something they even could purchase or donate to. I do think the VCA is slightly different from a lot of other anatomy resources in that it is much more interactive, constantly updated, and represents years worth of work on the part of those involved, and I think it was probably frustrating to them knowing that it couldn't be improved further without more monetary buy in from those using it. I wonder if putting it behind a paywall was just the next step in the program development process for them, but I'm not sure.