Why Use Interfaces Instead of Abstract Classes With DI?

I am beginning my journey of learning about dependency injection, and one of the reasons that I saw why it is a good idea to use DI was that it explicitly specifies your dependencies, which also makes your code more clear.

I have also noticed that interfaces are used abundantly, but I want to know why would we not use abstract classes for the sole purpose of specifying a default constructor?

Of course no implementation could be included in the abstract class.

Wouldn't this:

abstract class FooBase
{
    protected IBar _bar;

    FooBase(IBar bar)
    {
        _bar = bar;
    }

    abstract void DoSomething();
    abstract void DoSomethingElse();
}

Demonstrate more clearly what the dependency of a FooBase object is more than:

interface IFoo
{
    IBar Bar { get; }

    void DoSomething();
    void DoSomethingElse();
}

?

Please keep in mind I am new to this whole concept so be nice :)