Why Laylatul Qadr is hidden??

Shaykh Muḥammad Ṣāliḥ al-'Uthaymīn رحمه الله said: "Allāh has kept it (knowledge of Laylatul Qadr's occurrence) hidden for two great reasons:

1. To distinguish the truthful one in seeking it from the lazy one, because the truthful one in seeking it does not mind to strive for ten nights in order to attain it, whereas the lazy one is lazy to stand in (prayer) for the ten nights for the sake of attaining one single night.

2. The Muslims can attain more reward by doing more (good) deeds, because the more (good) deeds they do, the more the reward.
[تفسير جزء عم ص ٢٧٧]