Because if it actually was genetic then EVERY child born to a mage couple would be a mage too. However as it is, that is not case.
That's not how genetics work. Please, take a look at Mendelian inheritance:
http://en.wikipedia....ian_inheritance
Recessive alleles may be masked by dominant alleles. And every individual has a pair of alleles for each particular trait.
Let's suppose that magic comes from just one trait. Highly unlikely, but let's assume it for the sake of this mental exercise. To put things more in the mages' favour, let's also assume that magic is dominant. Well, if both parents have a pair of magic alleles, then their children will have too. However, if the parents have recessive alleles masked by dominant magic alleles, their children have a 25% chance of being born muggles.
And that is in the best case scenario. Magic alleles could be recessive instead of dominant, or you could need a certain combination of genes to produce magical offspring. So yeah, magic could depend on genetics easily. What it doesn't explain is the 'mutation', that is, that people with no mage ancestor can be born as mages. And a very specific mutation, indeed. Unless a certain interpretation of Sandal's prophecy is true and everybody was a mage in the past (hey, it could be genetics too: maybe the Maker switched the mage alleles from 'dominant' to 'recessive' all over the world).