Why did Allah Almighty say, "There was for you a reward" when describing the bliss of Paradise, and not, "There will be for you"?

The discourse in the verse is addressed to the inhabitants of Paradise after they have entered it and witnessed its delights. They are told: "Indeed, this was for you a reward, and your effort has been appreciated." That is, the blessings mentioned were a recompense and reward for their deeds, and their actions in the world, in obedience to Allah, were pleasing and accepted. Some exegetes are of the view that the discourse is addressed to the believers in this world, and that the mentioned blessings were, in Allah's knowledge and judgment, a reward for them.