There are a couple of reasons that blends are more readily available than B-100.

First.
No engine maker recommends greater than B-5. That has to do with some still unanswered questions about the long term consequences of using BD, there just hasn't been enough research done to be certain it is safe for the engine. B-5 blend meets the current ASTM standard for regular (Petro based) diesel so engine makers consider it safe. Some engine makers will tacitly approve B-20 but warn that warantees will be void if any problems can be traced to the use of it. Standards are currently being developed aand research is in progress to answer these questions.

Second
BD has a higher "cloud point" than regular diesel, that means it will crystalize and start plugging up filters at a higher temperature, which can cause problems in colder parts of the country. This problem can be resolved by blending in some reg #2.

Third
The BD production capacity, in the US, is still in its infantile stage. It would be impossible for it to keep up with demand if everyone started using B-100.

Note
As clean air regs demand cleaner burning fuel you may see an acceleration in the use of BD. It decreases CO2 emissions, reduces particulate emissions, and it is extremely low sulphur yet lubricates better than high sulphur content fuel.