Evacuations are for imminent danger (ie- where the hurricane makes landfall). The damage is fast and intense. What Houston, Spring, and other areas are experiencing is a build-up of flood waters. The post-hurricane rain is unpredictable and unfocused, so it's difficult to be sure which neighborhoods are in danger. Spring is a LONG ways inland, yet they are experiencing flooding also. It is impossible to safely relocate an entire population center the size of Houston and its surrounding suburbs - especially when you don't know where the flooding will be the worst.
