Because not all bats want to live in Central Texas.  Some of them prefer the Gulf. And many bats love being in the center of the city.  Take the Waugh Bridge bat colony for example.  The bats live in the crevices of the bridge year round, and the best time to see them is in the Summer.

Given how temperate it is in Houston, you can really see them emerge year round, so don't feel bad if you're in town for an "off" season.  In fact, while the colony on Waugh is the largest, there are smaller colonies located close to the bayous in Houston. 

The emerge at dusk, but as with any other type of wildlife, they don't work on a clock.  Any day you come by to watch, there's sure to be a crowd to watch them emerge with you.