Why didn't they evacuate?

As always, there are many many reasons not to evacuate.  It's never a clear cut decision for an individual. Where will you go? How will you get there? Can you afford to go? Are you healthy enough to go? Can you afford to stay there? How sure are you of being able or allowed to come back? Also, what if evacuating literally kills you?

Houston area officials who urged people to stay home before the storm may have been remembering that the city government was strongly criticized after the disastrous evacuation before Hurricane Rita in 2005.

In the hours before Rita struck the Houston area in September 2005, government officials issued an evacuation order, and some 2.5 million people hit the road at the same time, according to the Houston Chronicle.

More than 100 people died in the mass exit from the city — almost as many as were killed by the hurricane itself.

Dozens were injured or died of heat stroke waiting in traffic for nearly a full day. Fights broke out on clogged highways. A charter bus carrying people from a nursing home exploded on the side of Interstate 45, killing 24 people inside

Therefore evacuation shouldn't be a clear cut decision for public officials to make. There's no such thing as a "mandatory evacuation." Or, at least, we shouldn't be thinking in those terms.  Give people the best information and advice. And, by all means, do whatever  is possible to help them go if that's the best thing.  But it's not possible or helpful to make them go.  It's also not helpful to blame them if they choose not to.