Why, because biofuels are carbon neutral i.e. they absorb the co2 when you grow them then release it when you burn them. Repeat.
Whereas the black stuff just releases. The official argument against is the fear south American countries would chop down rain forest to grow it, releasing stored carbon and destroying carbon absorbers. Instead they chop down rain forest and farm cattle which produce more green house gasses.
What also seemed to worry politicians was the prospect of countries that currently struggle to feed their population i.e. Africa and India would switch from food to biofuels production & export if it became more economically viable to do so.
Or, that in places like Europe we would switch to biofuel production and import our food from South America / New Zealand etc with the obvious carbon footprint.
Still not convinced it is not viable or Europe to grow it with the addition of some import taxes. But then we get political not rational.