I'll take a swing at this.. one reason is that modern electronics contain - both directly and through remote access to other systems via stored cookies, etc - far more private and/or commercially-sensitive information than almost any other widespread physical object that may cross a border. In the 1960s a (non-spy) traveler faced a search of some clothing, cash-on-hand, and perhaps some food/snacks and prescription drugs. Far different from facing a search of every photo you've ever taken, every private message you've written to your spouse, or the full IP of your successful software company, etc.