Why are the events of Revelation 6 called the beginning of travail?

Revelation 6 events are called the beginning of travail because they are only the first, relatively mild birth pains of the coming tribulation, compared to the far greater judgments at the end. They start the delivery process, with Jacob's trouble ultimately focused on the nation of Israel.

Les explains that Jesus was referring to all the events coming on the planet, beginning with the horsemen of Revelation. These are just the beginning, like a mother about to deliver: her birth pains at first are relatively mild and far apart. As great as these events will be, they are mild compared to what is coming at the end. So these things are just the beginning of travail, the beginning of the delivery process. Les also points to Jeremiah 30:7, where the day is great and none like it, the time of Jacob's trouble, showing these things will have their vortex upon the nation of Israel.

What did Jesus mean by the 'beginning of travail' in relation to the Revelation horsemen?

The “beginning of travail” means the Revelation horsemen mark only the early labor pains of coming planetary events. Like a mother’s first contractions, these judgments are relatively mild and far apart, yet they initiate the delivery process that will intensify until the end, with special focus on Israel's tribulation.