The simple answer is money. Before Pip's acquisition of wealth, Trabb and Pumblechook didn't give Pip a second thought, and even treated him with a bit of derision. When Pip comes into his money, Trabb and Pumblechook decide that he might be worth their time. Pip goes into Trabb to buy some new clothes, and remarks that Trabb "did not think it was worth his while to come out to me." At this point, Trabb did not know that Pip had money; he was eating his breakfast and didn't even stand up to help Pip. However, when Pip tells him of his wealth, Trabb immediately sets down his breakfast and Pip remarks, " a change passed over Mr. Trabb." Trabb then goes out of his way to behave respectfully. The only difference here was the fact that Pip had money.
It is the same with Pumblechook. Before Pip's money, Uncle Pumblechook was a bully to Pip; he teased him, scorned him, called him names, and beat up on him a little. After Pip received his money, Pumblechook changed his tune. All of a sudden Pip was "my dear boy," and "my dear young friend." He becomes submissive, asking Pip's permission for everything, and goes out of his way to feed him and be polite and solicitous. Again, the difference here was Pip's money.
It just goes to show how much power money can have; it also shows a rather unflattering side of human nature, one that indicates we will do quite a bit for money. I hope that those thoughts help; good luck!