The name of the base class must come before any interface. Why?

Enclosed code would give you the "Class must come first before interface" error. I understand it is a rule, but can anybody tell me what the reason behind it?

public interface ITest
    {
        int ChildCount { get; set; }
    }
 
public class Test
    {
    }  
 
public class OrderPool : ITest, Test
    {
        public int ChildCount
        {
            get;
            set;
        }     
    }

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