Why does the seven-year tribulation have to happen?
The seven-year tribulation must occur because the prophetic program spans 490 years. It is a required part of that timeline, not optional. Though it could have begun soon after Christ's ascension, it still had to run its course so Israel could receive her king.
Les explains that the seven-year tribulation has to happen because the prophetic program is 490 years in length. He notes it could have unfolded within just a few weeks after His ascension, run its course, and Israel could have had her king. The disciples asked the Lord if He would at that time restore the kingdom to Israel. He did not upbraid them or deny the concept; He only refused to answer the when, saying it was not for them to know. Thus the tribulation is a required part of the prophetic timeline.