Update from CNN. It wasn't a stuck rental car. The car got them to the trailhead just fine. It was attempting a 4.6 mile, no shade, deep sand hike in 101F (38C) with only 40 oz of water for three people that killed them.

http://www.cnn.com/2015/08/08/us/new-mexico-french-deaths/index.html

The Inquisitr has a picture of the couple. They don't appear to be out of shape. They just badly underestimated the affects of the desert sun and the dunes.

http://www.inquisitr.com/2319070/french-...im-their-water/

I've gone out in late season desert hikes with (4) 1.5 L bottles (>200 oz), and come back to the car 4 hours later with less then a liter remaining. It's incredibly easy to underestimate water needs in the open desert.