Because Isaiah 9v6 does not teach that God become a man. You're in a tough position here, because to make any kind of a case for this, you have to show me why you think Hezekiah is not God-incarnate and also why translations have the singular word "name" in that verse and then procede to give 8 seperate names. The fact of the matter of the matter is, the name (singular) is precisely that - one name - Pele-joez-el-gibbor-abi-adar-sar-shalom. This translates into English as the one name "The Wonderful Counsellor is the Mighty God and is the Everlasting Father of the Prince of Peace". In other words, Jesus was to be given a name which identifies him as the son of the Mighty God.
Let's look at the Hebrew translation before you start juming to conclusions pal.
For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace. Of the increase of his government and peace there shall be no end, upon the throne of David, and upon his kingdom, to order it, and to establish it with judgment and with justice from henceforth even for ever. The zeal of the LORD of hosts will perform this.
Isaiah 9:6-7
6383. pil'iy, pil-ee'; or paliy', paw-lee; from 6381; remarkable:-secret, wonderful.
2. 6382. pele', peh-leh; from 6381; a miracle:-marvellous thing, wonder (-ful), (-fully).
Strong's Concordance
The Angel of YHVH appeared to [Manoach's wife], and said to her, "Behold now, you are barren, and bear not: but you shall conceive, and bear a son. . . the child shall be a Nazarite unto God from the womb: and he shall begin to deliver Israel out of the hand of the Philistines. Then the woman came and told her husband saying, 'A man of God came to me, and his countenance was like the countenance of an angel of God, very awesome . . . he did not tell me his name. . . And Manoach said to the Angel of YHVH, "What is your name, that when your sayings come to pass, we may honor you?" And the Angel of YHVH said unto him, "Why do you ask my name, seeing it is Wonderful?1"
" . . . and Manoach said to his wife, "We shall surely die, because we have seen God."
Judges 13:3,5-6
The only difference between the name 'Wonderful' in Judges 13, and the occurrence in Isaiah chapter 9, is that the name in Isaiah 9 is missing a yud. The letter yud is a symbol of Divinity, as it is the first letter in the Name of God, YHVH, yud-heh-vav-heh. Yud is the "Y" of the Hebrew language, and occurs in many Biblical names such as Yeshua, Hezekiyah, and Y'hoshaphat. What connects these two words, Pele (Wonderful), and Pil'iy (Wonderful, Secret), is that these are the only two times in the entire Tanakh, that the term is applied as someone's name. The two names here are referring to the same Person.
The dropping of the yud in the reference to Isaiah 9 is a picture of the Incarnation of the Messiah, who emptied Himself (while never changing His nature), of His Divine Attributes, as it says in Colossians 2,
Let this mind be in you, which was also in Messiah Yeshua: Who, being in the form of God, thought it not robbery to be equal with God: But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men: And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross. Wherefore God also had highly exalted him, and given him a name which is above every name: That at the name of Yeshua every knee should bow, of things in heaven, and things in earth, and things under the earth; And that every tongue should confess that Yeshua the Messiah is Lord, to the glory of God the Father.
Phillipians 2:6-11
There is no reason why we should take El in this name of the Messiah in any other sense than in Immanu-El; not to mention the fact that El is Isaiah is always a name of God.
As for Hezekiah he's been dead and in the ground long before Isaiah gave that Prophecy. You appeal to tense in Hebrew grammar, Hebrew verbs do not take past, present and future forms, but perfect (completed action) or imperfect (incomplete action). Isaiah 9:6 uses special idiom in Hebrew called the "Prophetic Perfect" this is where a prophet speaks of future events in the perfect form because he has seen them in the future where they have already happened.
If you don't know Hebrew I am going to have a field day refuting you. NICE TRY, what's next.
By the way thanks for answering my Question; took you a while but you did it.