Interesting points have already been served up on this topic. But working in Los Angeles for almost a decade now has given me a closer perspective. One of the issues is SF, Boston and NYC are served by prominent Landscape Architecture programs at Berkeley, Penn, Harvard etc. These programs offer opportunities to connect with a large percentage of the publishers and editors of publications and books that feature the newest, “hottest” work. LA doesn’t have the academic community that you find in NoCal and the Eastern Seaboard. Although USC is committing resources to build a MLA (and future BSLA) program to their renowned School of Architecture.
LA also has a design culture that is architecture heavy. Project teams are assembled by architects, which results in landscape architecture becoming a part of a larger design composition focused primarily around buildings. This provides opportunities for a great number of firms, large and small, to take part in work and thereby reducing the influence of a select few groups. Also, the urban quality of our site conditions (yes, LA is quite urban) make for compact and efficient designs, reducing the opportunities for large landscape design moves that photograph so well in those glossy sheets of LAM and the like. However, they make for great innovative and well detailed work.
Only recently have there been opportunities for open space design work in the city led by landscape architects. We’re looking forward to more in the future. Melendrez recently completed the Santa Monica College Quad in Santa Monica, Rios Clementi Hale has designs for a civic park in downtown LA that is waiting on funding (hope, hope) and Mia Lehrer + Associates opened Vista Hermosa Park near downtown LA a couple of months ago (the first planned and designed new park in LA in nearly 100 years!)
There are very innovative, environmentally focused, design oriented firms in Los Angeles, some of which have been mentioned already. To add to the list: Melendrez, EPT, SWA and EDAW are represented in Southern California as well.
Hope this helps.