In theory, the idea of using high ground maps sounds great, but there's high ground, and then there's "high" ground.
I live on "high" ground. A flood map would show as much, and I have the lower insurance premiums to prove it. In New Orleans, that means my house is 3 feet above sea level (which is quite high for the city). However, it doesn't mean the neighborhood won't flood. It just means that the water's unlikely to rise high enough to enter the raised houses.
I grew up in the desert. When I first moved here, I had no experience of flooding and no idea what to expect. Katrina hit the next month. As the storm was bearing down on us and I was trying to decide whether to evacuate or not, I asked my elderly neighbor whether the neighborhood we lived in was likely to flood. She responded "Oh no. It doesn't flood here. The water hardly ever gets more than knee-deep."
I thought to myself, "Well, shoot, that sure as heck SOUNDS like flooding."
I weathered the storm at a friend's house, and after it had passed, I waded through three blocks of ever-deepening water to check on that neighbor. Upon arriving, I noted that the thigh-high water was lapping the top step of the porch, four inches shy of the house itself, and I understood.
Now, after 12 years, I'd likely give the same response if a new neighbor asked about whether my street would flood. "Here? No, we don't flood. The water hardly ever gets more than knee-deep."
High ground is secure and dry. "High" ground means your car will still be ruined if it's parked on the street, but your house will probably survive. It also makes a lousy evacuation spot, because you're stranded until the water goes back down, deliveries of supplies would have to be made by boat, and an extra few inches of water (or a dickhead going a bit too fast down the street in a motorboat) marks the difference between high-and-dry, and water sloshing around your cot.
Houston has plenty of "high" ground, but it doesn't have a lick of proper high ground.
ETA: That's not to say there aren't preventative measures the city could take. The discussion of changes to building codes is back on the table. They could even, theoretically, build all new public structures with the intention of having them double as shelters. Adding more greenery to help absorb water, etc. etc. It's just not as simple as telling people, "go to this area and you'll be safe." I'm not saying it will continue to be that way. You seem to be under the impression that everyone's just throwing their hands up and crying, "there's nothing we can do, so we'll do nothing." That's not the case at all. It's just that, for this storm, with the city as it exists now, there was no good option. I'd imagine when they start rebuilding, they'll do so with an eye to preventing a recurrance of this mess.