I only read the first three or so pages of this thread but I felt the need to chime in.
There are a number of factors that I feel contribute to the fact that non-white Americans are not widely represented in American cycling.
My parents are from Mexico, as is a great deal of my family, and I have experienced a few things that may shed some light on why there are not too many Mexican-American cyclists.
First, the majority of my family thinks of cycling as a good form of exercise and a decent method of transportation...for very short trips, anyway. Many of my family members simply do not understand, or care to understand, cycling as a sport--recreational or professional.
Also, I feel that the appearance of cycling clothing may turn some outsiders off of the sport. They simply cannot begin to comprehend why any male would willingly wear spandex, tight fitting jerseys, special shoes, a helmet and sunglasses to ride a bicycle.
Then comes the price factor. Cycling is an expensive sport in comparison to sports like baseball, soccer, basketball, and football. Those sports require widely available inexpensive equipment and a group of friends. To participate in any of those sports at a recreational level requires an investment of under $50 in most cases. Whereas cycling requires at the very least a bike and a helmet. I would argue that to fully enjoy cycling [read: comfort], it would be wise to invest in some clothing as well. The cost of all of that combined can be well over what a person may be willing to spend in a sport, regardless of their income. I have had some well-off family members tell me that they would never spend over $100 on a bicycle, part or accessory because they can be had for that price [or cheaper] at big box retailers.
Which brings me to my final point, it comes down to exposure. If cycling had as much exposure as all of the other major sports in the U.S., we would not be having this discussion. Regardless of the cost of entry into the sport, more people would be willing to participate.
This relates to track and road cycling by the way. In regards to BMX and mountain biking, I always hear people refer to them as "kid sports."