Why did John reject the Sadducees and the Pharisees?

The Sadducees and Pharisees were curious about John's teachings and came to see his baptisms, but they chose not to repent and to be baptized. When he saw this, he chastised them and called them to repentance. His message must have angered them.

He warned them that they must repent and told them if they didn't they would be rejected as the children of Abraham. They would lose their inheritance and not receive eternal life. They would be cut off at the root and thrown into the fire. Then, his most important message was of the coming Messiah, who he said would baptize with fire and the Holy Ghost. This message they rejected and did not understand. This is what he said to them as recorded in Matthew 3:7-12:

7 O generation of vipers, who hath warned you to flee from the wrath to come?
8 Bring forth therefore fruits meet for repentance:
9 And think not to say within yourselves, We have Abraham to our father: for I say unto you, that God is able of these stones to raise up children unto Abraham.
10 And now also the axe is laid unto the root of the trees: therefore every tree which bringeth not forth good fruit is hewn down, and cast into the fire.
11 I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire:
12 Whose fan is in his hand, and he will throughly purge his floor, and gather his wheat into the garner; but he will burn up the chaff with unquenchable fire.

His message, however, was received by the publicans, who were tax collectors under the Roman rule. (See Luke 7:29-30)

Who were the Sadducees? (see http://scriptures.lds.org/en/bd/s/7)

The Sadducees were a party or caste among the Jews, originating at the time of Solomon. They included the families of high-priests who came forward and defended Judah in the Maccabean war. Although they were few in number, they were powerful and formed the Jewish aristocracy. They followed to the letter the Mosaic law and denied the authority of ancient tradition, thus opposing the Pharisees who believed in angels and spirits and the doctrine of immortality. Through their influence, Greek culture spread in Israel. They were opposed to Jesus Christ and the work of the apostles because they preached the resurrection (Acts 4: 1-3; cf. Mark 12: 18-27; Acts 23: 7-8).

Who were the Pharisees? (see http://scriptures.lds.org/en/bd/p/30)

Another religious party among the Jews, the Pharisees "prided themselves on their strict observance of the law, and on the care with which they avoided contact with things gentile." They believed in the doctrine of immortality and resurrection of the body and the existence of angels and spirits. They regarded oral tradition as of equal value with the written law. The word Pharisee means "separatist" and, because their strict observance of a multiplicity of ceremonial rules, they separated themselves out from the rest of Judah and considered themselves better. Their opposition of Christ prevented many Jews from accepting his message. For the Lord's judgment on them and their works see Matt. 23; Mark 7; Luke 11: 37-54.