Dickens uses the characters and situations in the book to make a pointed social commentary, attacking the hypocrisy and flaws of institutions, including his society's government, its laws and criminal system, and its methods of dealing with poor people. Dickens basically believed that most people were good at heart but that their good impulses could be distorted by social ills.
Throughout the novel, Dickens uses Oliver's character to challenge the Victorian idea that paupers and criminals are already evil at birth, arguing instead that a corrupt environment is the source of vice. Oliver was well spoken compared to the cockney scraggamuffins in the gang. He was also 'appalled' that they actually robbed people for a living.
We must also remember Dickens� audience. To us, his social criticism might have been more effective if he had focused on a more complex poor character, like the Artful Dodger, Fagin or Nancy, but the audience may not have approved (a thief, a jew and a prostitute?). Dickens's Victorian middle-class readers were likely to hold opinions on the poor (and Jews) that were only a little less extreme than those expressed by Mr. Bumble, the beadle who treats paupers with great cruelty. Oliver Twist was criticized for portraying thieves and prostitutes at all. Given the strict morals of Dickens's audience, it may have seemed necessary for him to make Oliver a saint-like central figure to appeal to the masses. Making Fagin a central character � albeit covertly � rather than a low class thieving Jew would probably have lead him to be strung up himself.
The Victorians loved the idea of a poor little rich boy being abandoned and abused (against his better morals) to the wiles and whim of the lower classes and "crafty" Jew population, and then finally bought back from his own odyssey to his 'proper' societal status. That being, Oliver, probably was the central character at the time.