Why Dictionary is preferred over hashtable in C#?

var customers = new Dictionary<string, Customer>();
...
Customer customer = customers["Ali G"];

var customers = new Hashtable();
...
Customer customer = customers["Ali G"] as Customer;

[Serializable, ComVisible(true)]
public abstract class DictionaryBase : IDictionary, ICollection, IEnumerable
{
    // Fields
    private Hashtable hashtable;

    // Methods
    protected DictionaryBase();
    public void Clear();
.
.
.
}
Take note of these lines
// Fields
private Hashtable hashtable;