The excuse usually given is that Mormon's can't afford to live there ("there" being any location in California) anymore, although the area population has almost always gone up, so *some people* seem to be able to live there.
I expect most LDS youth in California during the Prop 8 debacle, and perhaps a fair number of adult members, were embarrassed, perhaps even humiliated to be identified as Mormon, and bailed out when an opportunity presented itself.
Given that the mormon church routinely uses inflated membership numbers, it would be hard to report a large ward of 400 members when only 13-14 show up.
Again, success is whatever they claim. Like when the Church News cheerfully announced the organization of 3 new stakes. The new stakes, however, were created from the consolidation of 5 stakes, for a net loss of 2 stakes. I'm sure that the faithful Mormons were ecstatic about the 3 "new" stakes, and all the "progress" they represented.
I am also very connected to the closure of the Catania, Italy, mission, due to, according to the church, they enormous success they've had in the region, making a mission unnecessary. Seriously!, this is how they couched the sad information. Italy has gone from one mission to two missions to three missions, to four missions, to three missions, and back to two missions. I fully suspect that it will go back to one mission in the not-too-distant future. There are also no longer missions in Ireland, Netherlands, Austria, and Switzerland; all those members are now served by missions outside their countries. Engh, somehow, this doesn't speak "success!" to me, personally.
Camarillo CA Stake had 11 wards just 10 years ago, 8 English, two Spanish, and a YSA ward. Today the total 5 and a half, 4 English, one Spanish and the YSA ward which is now shared with the Ventura Stake. A member of the SP told leaders that California is losing 5000 members a year to moving to cheaper places.