"Double" doesn't get cast as "IComparable<Double>"

I have created a template that returns the index of the max value in an array. It works, but only if I pass in a weirdly cast List.

    static public int FindMaxIndex<T>( IEnumerable<IComparable<T>> arr )
    {
        IEnumerator<IComparable<T>> it = arr.GetEnumerator();
        if (!it.MoveNext()) return -1;
        int index = 1, maxIndex = 0;
        IComparable<T> max = it.Current;
        while (it.MoveNext())
        {
            if (max.CompareTo( (T)(it.Current) ) < 0)
            {
                maxIndex = index;
                max = it.Current;
            }
            ++index;
        }
        return maxIndex;
    }

Now to use it:

    List<IComparable<Double>> arr = new List<IComparable<Double>>(); // THIS WORKS
    List<Double> arr = new List<Double>(); // THIS DOESN'T

The later list, which is what I would like to use, gives this compiler error:

cannot convert from 'System.Collections.Generic.List<double>' to 'System.Collections.Generic.IEnumerable<System.IComparable<double>>'

How can this be? "Double" IS an IComparable; taken from its definition:

public struct Double : IComparable, IFormattable, IConvertible, IComparable<Double>, IEquatable<Double>