1 They're tough to design well

It may not seem like a huge engineering challenge to make a door open up instead of out once you have the idea, but there are a few persistent design challenges that plague gullwing doors in particular.

The first is that the weight of the door, which is generally pretty heavy, has to actively fight gravity on its way open instead of just pivoting on a traditional hinge. Fortunately you don't have to just force passengers to just lift the whole weight of the door themselves. One solution (which the DeLorean uses) is to use a torsion bar to help, a rod that's twisted to create tension when the door is closed, and that untwists and helps to lift the door once it's opened. And like the Tesla X, you can also just power the doors.

But that's not all. Green Car Reports—which dug up a handful of of-the-record sources to talk about the Tesla X in particular—surfaced a number of other challenges earlier this year. You have to make sure the roof isn't going to warp at all. You have to find room for the torsion bars and power assist in the small part of the roof that is not part of the door. You have to make sure they are properly sealed. Gullwing doors have a tendency to be a bit leaky in the rain. Elon Musk himself mentioned it in an earnings call, as Green Car Reports points out, assuring that the company "has learned a lot about door seals."

These aren't insurmountable engineering challenges, but they are challenges you can get around by just using traditional doors instead.