Received rejection by UCLA /USC, and wait-listed by UCSD. With a 1550 SAT Why????

Well, it is difficult to say with this information alone. Most competitive public/private schools have a formula that normalizes your application. Typically it follows the pattern below with slight variations from school to school along with cut off.

AI (Academic Index) and PAI (Personal Achievement Index) are HUGE indicators on a prospective students potential acceptance to a given University.

For context on the "UT required high school curriculum" This can be substituted for any major State/Private University.

The high school curriculum standards are as follows:

The Recommended Program requires 26 credits. Students must complete four credits in each core area: English, Math, Science, and Social Studies, two credits in the same world language, one credit in PE and Fine Arts, a half credit of speech and five and a half credits in electives.

So, to answer the question, just 1550 on the SAT does not cut it if the other variables are poor. Review this list and assure that each bullet point is covered.