Most people did evacuate. Those who didn't were folks who had no way to leave. Or do you imagine that people just watched the water rise around them and decided to just save their gas for something TRULY important?
See, I know folks who didn't even have a reliable car, but drove as far as their automobile would take them. I remember the folks whose cars broke down on the highway in the 12-16 hour wait to reach Baton Rouge. Some of these folks were so desperate to get out that they started begging and offering money to any random stranger that would agree to at least drive their children or elderly parent to the first safe spot out of town that their money could convince the person to agree to.
Now, perhaps you have no sympathy for the elderly who died in their attics, scratching their fingers until they were bloody in an attempt to open some kind of hole that might have allowed them to get on top of their house. Perhaps you have no sympathy for those post-surgery patients in intensive care at hospitals that couldn't convince any medical facility to let them send these patients to safer areas.
Perhaps you know nothing about the folks out in St. Bernard Parish who didn't even have running water, much less cars to take them out of the danger zone. Hell, some of them can barely speak anything other than French! They couldn't even understand the evacuation order unless they had a bilingual relative or neighbor to explain it to them. None of the ignorant, but mouthy, outsiders are even aware of the fact that St. Bernard Parish suffered more damage than New Orleans. The Cajun white people out there are still pretty pissed at y'all Anglos about that.
Perhaps you have no sympathy for the families of first responders who believed that, while they were risking their lives for others, the American government would at least make sure that their own families were taken care of. After all, it's not like there were any hotel rooms available out of town. All of the hotels and motels in Baton Rouge, LaPlace, and Lafayette were packed to 100% occupancy even before the evacuation order was given.
Of course, we don't expect folks like you to care about your fellow Americans. Y'all know nothing about what it means to "Love thy neighbor". Say what you will about the black people of the Gulf region who were labeled "refugees" and the white people who were considered not-quite-white-enough to be worth saving. However, I'd rather spend my life surrounded by any person down here, before I'd live with people like you as neighbors. At least we help each other down here. Folks like you would crumble in an instant if you faced these problems.