Why does [FromBody] deserialize data into properties and not fields?

Question

ASP.NET

For example, there is a controller accepting stringfy json

public ViewResult Insert([FromBody]InsertGroup json)
        {
             //Some code
        }

And the class in which the deserialization takes place

public class InsertGroup
    {
        public int[] selnames { get; set; }
        public string template { get; set; }
    }

If instead of auto-properties there are regular fields, the data simply will not be recorded. Why is this happening?

Answer 1

I suspect it has something to do with reflection. In any case, you can see the source code and create your own binding.
PS why don't you like the properties?