Obviously one reason is due to high costs and the fact that the US has more oil than Saudi Arabia which ensures continued low cost fossil fuels unless they are regulated and taxed out of existence. At that point, as in the case of coal producers, forcing energy prices higher will make biodeisel more affordable. Also, there is the obvious issue relative to the deisel's popularity except in the commercial transport market.
Furthermore, there are unintended consequences when using "food" as a fuel. Due to limited scarce resources, such as land, food supplies and the cost of food is adversely affected when competing against the energy sector. Hence, not only does this result in higher food prices at the grocery store, but it also leads to higher fuel costs.
One reason is that "oil, coal, and natural gas" doesn't have an alternative use as a food. Hence, it is either used as a fuel as intended or remains an untapped useless natural resource. Another affect NOT using fossil fuels has economically the costs of all other goods and services increase, thereby driving down the economic growth and standards of living.
Biofuels are costly, inefficient energy creators, and a poor use of scarce resources especially when taking productive farmland out of food production.