The best of both breeds?  Why would crossing two breeds of dog result in only the best features of each breed?

All breeds have their good features and their bad.  Individual dogs within each breed can display desirable and undesirable traits.  Nature does not take all the good bits from each parent to produce the perfect dog.  Even when breeding 2 dogs of the same breed this doesn't happen, otherwise all pure bred dogs would be perfect!

Undesirable traits and features have a habit of dominating the good and can stay hidden for several generations . . . . . before rearing their ugly head.  Things like health problems, bad temperaments etc may suddenly re-appear when two dogs of different breeds are crossed.  Each mating is a risk as there is no benefit of hindsight or experience that a breeder of pedigree dogs will have.

A pedigree breeder will have good knowledge of the lines behind their dogs adn will have bred selectively away from the undesirables.  With a cross breed you are just as likely, if not more so, to see the worst of each breed rather than the best.