Habib Ali al-Jifri (may Allah protect him and benefit us by him) explains.
He could have made it a specific night so people could be completely prepared for it.
But it is concealed in order that the believer remains in a state of anticipation – waiting and hoping for the bounty of Allah. Thus his connection to Allah is not based upon one specific time which comes and goes and then he turns away from Allah and gets on with something else. This is the divine wisdom.
Furthermore, that anticipation of Allah’s bounty nurtures a state of knowledge of Allah in the heart – the more you turn to Him, the more you seek His pleasure, the more you hope for His gifts the more light increases in your heart. Allah says in the hadith qudsi: “I am as my servant perceives Me.”
The more you think good of Him, the more goodness grows in your heart; the more you believe He will give you, the more He will give you.
This develops and nurtures your relationship with Allah which is the secret behind Laylat al-Qadr, the secret behind Ramadan and the secret behind every act of worship.