Why the Doomsday is kept secret?

There are vast explanations about the Doomsday, which has primal importance for monotheistic religions both in Quran and in Hadith books. Humans are warned to be prepared that the Doomsday is so immanent and so devastating.

In a hadith which narrated by Anas bin Malik, He heard Prophet Muhammad as saying, I and the last hour have been sent like this and (he while doing it) joined the forefinger with the middle finger. (Muslim, Book 041, No 7049)

Prophet Muhammad (peace be upon him) with these words was referring that the last hours imminence.

In Quran, the statement, Doomsday is so immanent, and Prophet Muhammads (pbuh) announcement that the last hour would come soon should not bring any suspicions to the minds just because it has already passed 1400 years and did not happen yet. Because, Doomsday is directly related with the life of the world. Doomsday is appointed hour of the world, and in relation to the life of the world one or two thousand years are like one or two minutes in relation to a year. The hour of Doomsday is not only the appointed hour of mankind that it should be related to it and seen as distant. (Risale-i Nur Collection, Bediuzzaman, Words, 24. word)

It may not have been given the exact time of the Doomsday neither in Quran nor in Hadiths but Prophet Muhammad (pbuh) by pointing out the distinctive signs in Hadiths and in Quranic verses, which will happen near to the last hour constantly warned the believers for the appointed day.

Reasons and wisdom in the hour of Doomsday being vague:
There are so many instances of wisdom of it. It is as if one knows his time of death and being left destitute of belief and Islamic consciousness

Calamities and disasters vagueness has the same wisdom. Because waiting for the specified disasters and calamities is much more torturing than experiencing it.

That is why numerous other instances of wisdom remain hidden as the Doomsday does.

Just as for individuals appointed hour of death, Doomsday for humanity is same. The author of Risale-i Nur Collection, Bediuzzaman explains the subject in his book Rays as follows:
If the appointed hour had been specified, the Early and Middle Ages would have been virtually unaffected by the idea of the hereafter, and they would have been lived in needlessness. And the humans of the last age would have no pleasure or value of worldly life because of waiting the specified last hour.

Also, if the death of the world had been specified, some of the truths of belief would have been clearly obvious and everyone would have affirmed them willy-nilly, as a result the mystery of mans accountability and the wisdom and purpose of belief, which are tied to mans choice and will, would have been negated, there will be no secret of the trial. It is for numerous benefits that the time of the Doomsday remains secret. Everyone therefore continually bears in mind both his death and continues life, and he works for this world and the Hereafter.

Doomsdays being vague will remain hidden until the crack of the doom. Whenever the sun rises from the west instead of east, the door of trial arena and repentance will be closed, every individual will believe in Doomsday. Nevertheless, it will be useless to be regretful or faithful by then.