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. Continue reading Why did Allah conceal Laylat al-Qadr?