Because no one knows they exact date and hour in which Jesus would come?
Wonder why the Lord would give a heads up to anyone who would listen as to the date Jerusalem will be desolate?1290 days in advance.
Three and half years is a good headsup!
And which year will that be when the 1290 days begin? Jesus does give us the timing of His coming as to what to watch for, but yet no date is given.
Parentheses added by me:
Matthew 24:29 Immediately after the tribulation of those days (God's great wrath during the seven trumpets sounding) shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken: 30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory. 31 And he shall send his angels with a great sound of a trumpet, and they shall gather together his elect from the four winds, from one end of heaven to the other.