Probably because of the massive power asymmetry between vendor and user.

Suppose my only option for earning a living depends on software from Vendor X, who has used lock-in techniques to eliminate their competition. At this point there would be nothing to be done except make vendor lock-in illegal.

It would be easy to do, too, simply by requiring either A. up-to-date, detailed documentation of file formats and protocols necessary for data migration, or B. support for an interchange format that provides the same features as a proprietary format.

I'm still not understanding why it should be illegal. We can start with an assumption that Vendor X eliminated their competition by fair practices or they would already be subject to antitrust regulations. If Vendor X uses their position to unfairly prevent competition (either in their own space or others), they are already breaking the law. If they aren't doing that, I fail to see why just having lock-in should be illegal.

This is not to say I'm a fan of vendor lock-in. Quite the opposite, actually. But I do know laws like this will always have unintended consequences, will always be manipulated by those same large companies ostensibly being regulated to give them advantages over new entrants, and, as a result, will always make the competitive landscape more difficult than it would have otherwise been. How big does your market have to be? How big does your company have to be? Is the broadband chip on your phone subject or just your whole phone? What about trade secrets?

The advantage of the antitrust laws is in the lack of specificity. It takes a complete trial to actually show wrong-doing.