I was wondering why many zoos on the west coast (specifically California) don't have larger indoor complexes (small mammal houses, nocturnal houses etc.) like many east coast zoos, but do have reptile houses.
Weather is the simplest answer. You tend to see more large indoor complexes in Northern zoos, whether that be on the East Coast (e.g., New York, Pennsylvania, New England), the Midwest (e.g., Ohio, Michigan, Illinois, Minnesota), or the west coast (e.g., Oregon, Washington). Large indoor complexes aren't common in the southern US whether that be Florida, Texas, Arizona, or California.
Reptile houses are the exception since temperature and humidity for herps (and more broadly all ectotherms) is typically much more important than it is for mammals and birds (not to say there aren't exceptions), and historically zoos have managed these species indoors since that makes climate control a lot easier. That's not to say they can't be housed outside successfully, but generally indoors is more common with herps due to temperature/humidity constraints.
Most herps are also much smaller than most of the small mammals that are common in zoos, and this could be another factor influencing why reptile houses are the "exception" to the fact indoor exhibits are less common in the south. In many other cases, southern zoos have an outdoor versions of indoor exhibits instead. For example, while many northern zoos have large bird houses or indoor aviaries (e.g., Bronx, Toledo, Cincinnati, Lincoln Park, Milwaukee), and many more feature indoor Rainforest attractions (e.g., Roger Williams, Franklin Park, Cleveland, Brookfield, Minnesota, Omaha), southern zoos instead often have large outdoor aviaries (e.g., Miami, Disney, San Diego), or outdoor areas with a Rainforest theme instead (e.g., Jacksonville, Disney, Los Angeles). There aren't as many examples of southern zoos, whether they be in California, Florida, or anywhere in between, that have outdoor reptile themed areas.