Did you give them a work email address by any chance? If so, that might be the data issue. I cannot think of any other reason why they would potentially change their mind (as it seems they were happy to provide it via email at first). The only thing could be if there are any data concerns relating to other people in the process (both interviewers or other candidates), but this seems unlikely.
Calls are generally a lot easier for them - it's quicker for them to do it over a call and answer any questions you may have, rather than providing a written document to you, and for you to then come back with questions and for there to potentially be back and forth. I generally encourage people to have a call rather than rely on something in writing as:
1) You can ask questions as you discuss the feedback
2) You can ask for clairty
3) You can listen to the tone a lot more - I think people will often read things differently when they are written versus how they may come across in a call
4) Although this doesn't sound like it would be the case in your instance, you could ask the firm what other opportunities there could be with the firm/whether you could reapply.