Why did the new ASP.NET Identity tables stop using Guid (uniqueidentifier type) as keys?
Identity is built to work on multiple storage platforms and not every storage platform has Guid as a supported storage type.
You can change the default string pkey into Guid, but that involves some work on your C# models. Or you can change the pkey into an int - whatever you like. Just be aware that there is a huge debate about which is better.