As the saying goes, speed does not kill. People don’t die just because they fly at twice the speed of sound. The rate of change in speed is what kills people. When you hit that Explorer, your car probably pivoted around the front corner and went sideways for a little bit before it skid to a stop. The impact slowed you down, but not enough to warrant impact deployment. If you were going faster or if you hit it squarely on the front (meaning no way for the vehicle to rotate in any direction), then the change in speed would be enough to warrant airbag deployment.
Also, like other posters said, wear your seat belt. Airbags are designed to fully inflate before you make contact with it. It acts as a air cushion in that manner. If you’re not wearing your seat belt, you are flying toward the airbag’s path of inflation. In that case, the airbag is a bomb to you.