Stacked BindableProperties not cascading

I am trying to make some custom ViewCells that have bindable properties in them. However, on the consuming page, if I try to pass in an additional bound value (e.g. to a ViewModel), the value is not passed in. Here is the test code to show the issue:

MainPage.xaml:

<?xml version="1.0" encoding="utf-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:ControlBinding" x:Class="ControlBinding.MainPage">
    <TableView Intent="Settings">
        <TableSection Title="Stock Cell">
            <EntryCell Label="Sample" Text="{Binding Message}" />
        </TableSection>
        <TableSection Title="Custom Cell">
            <local:CustomCell Label="Works" Value="Hello World" />
            <local:CustomCell Label="Doesn't Work" Value="{Binding Message}" />
        </TableSection>
    </TableView>
</ContentPage>

MainPage.xaml.cs:

using Xamarin.Forms;
namespace ControlBinding
{
    public partial class MainPage : ContentPage
    {
        public string Message { get; set; }

        public MainPage()
        {
            InitializeComponent();

            Message = "Hello Cruel World!";
            BindingContext = this;
        }
    }
}

CustomCell.xaml:

<?xml version="1.0" encoding="UTF-8"?>
<ViewCell xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" x:Class="ControlBinding.CustomCell">
    <StackLayout Orientation="Horizontal" Padding="16, 10" HorizontalOptions="Fill">
        <Label Text="{Binding Label}" />
        <Entry HorizontalOptions="EndAndExpand" Text="{Binding Value}" />
    </StackLayout>
</ViewCell>

CustomCell.xaml.cs:

using Xamarin.Forms;
namespace ControlBinding
{
    public partial class CustomCell : ViewCell
    {
        public static readonly BindableProperty LabelProperty = BindableProperty.Create("Label", typeof(string), typeof(CustomCell));
        public static readonly BindableProperty ValueProperty = BindableProperty.Create("Value", typeof(string), typeof(CustomCell));

        public string Label
        {
            get { return (string)GetValue(LabelProperty); }
            set { SetValue(LabelProperty, value); }
        }

        public string Value
        {
            get { return (string)GetValue(ValueProperty); }
            set { SetValue(ValueProperty, value); }
        }

        public CustomCell()
        {
            InitializeComponent();

            BindingContext = this;
        }
    }
}

Passing in the hard coded "Hello World" works, and is displayed. Passing in "{Binding Message}" results in nothing getting displayed.

Am I missing a binding nuance here?