I lost two back teeth. Why I have so much trouble speaking?

Air escaping. You would usually notice more trouble with speaking if you were to lose front teeth, but it is also possible you are noticing it with missing back teeth. Simply speaking, it is air escaping from mouth when that's causing difficulty speaking. Teeth have multiple functions, of which one of them is to "contain" sounds before it comes out of the mouth. Replacing these teeth would take care of this.
It will get better. We use our tongue and teeth to make many sounds. Tongue position, airflow etc, can affect our speech. Initially, after the loss of the teeth, it may be difficult to ennunciate certain sounds, but it will get better over time. Practice reading out loud, and you will adjust more quickly.