Because you get 60 gallons (270litres) of biodiesel per acre (out of soya beens)
Diesel is 18p/litre wholesale = �48 per acre
Ignoring turning the oil to biodiesel
Wheat say 7 Tonnes per acre - wheat about �100 per Tonne
�700 tonnes per acre
In order to make biodiesel profitable (and even feasible how many acres at 60 gallons an acre do you need?) you need to go from converting waste chipshop oil to growing crops with enough yield per acre to make actually growing the raw material profitable.
This report claims that algae could produce 15,000 gallons per acre - if that's true or even half true the possibilities suddenly become very interesting indeed - but it does seem an awfully big claim
http://www.i-sis.org.uk/GAFCCAB.php