Why? Because happiness fluctuates while bliss doesn't.
The key to get bliss is to understand mind. And it is not an easy thing to do. It takes time. It takes persistence. It takes determination. But it is possible.
We need to train our mind to be silent, to be calm. We need to tame our mind so that it serves us much better. To train our mind, the first thing we need to do is to watch the mind. Observe how it works, when it works actively. Then, you will see what it is and how it creates the world you live in.