I live in Norcal, and I have always wondered why the parks are closed in the winter too. Other than on rainy days (which we've had quite a lot of recently, but a lot less than most places nationwide), everyone pretty much walks around with a t-shirt on year round during the day. Nighttime is a different story though (we have to bundle up even on summer nights). It never gets "cold" here, just a little chilly. When I went to SFMW in October, it was great. There is nothing better than feeling that cool breeze on your face on the first drop. Las Vegas coasters are open year round, and it gets a lot colder there during the winter, so I don't think weather has anything to do with it.
The only reason why LA area and Orlando parks are open year round is because those are famous tourist destinations (although San Francisco is also). PGA and SFMW are more local parks. Less out of towners go there than, say IOA or SFMM. The management of Norcal parks figure the attendance wouldn't be high enough to warrant a year round schedule. Besides, it gives them more time to do maintenance.
I guess the same thing can be said for Texas parks? It doesn't get too cold there, does it?