Path of least resistance
According to a graph here http://www.bbc.co.uk/news/education-11012369 Computing is one of the hardest A'levels to get an A in (Maths being easier). And what do A-level points mean? Entry in a competitive world.
And most University Computing departments would rather see good passes in Maths and Sciences than the Computing A'level.
So why do Computing A'level?