Christians: Why is Jehovah not the name of God? ?

Because the word didn't exist till 13th-century Catholic clergyman Raymundus Martini made it up after he was commissioned to translate the Koine Greek Bible to Latin.

There was no "Jehovah" in the Bible until Tyndale mistakenly put it there in the 1500's.

"Jehovah" is a really poor germanic transliteration of "YHWH".

Because he doesn't have a given name as humans understand the term. There's no way to comprehensively understand his makeup and reality in the dimension he exists in, so he goes by his unique characteristics, in a way that transcends humans languages, times  and cultures.

It is the name of God, in one language. Every other language has their own name for God.

The name Jehovah is the English translation from Yahweh. People deny God's name for whatever they scheme in their heart for they do not attribute to God the power to place in men's heart what he wills. God says in his word that he will write his laws on their hearts, he also says he will put it into the hearts of the kings of the earth to carry out his one thought, even their one thought, to destroy Babylon the Great.They obviously don't believe that God could see to it that his name would be translated so that people may know that Jehovah is the Most High over all the earth, Psalms 83:18. Jehovah is Judge and we'll see how it goes for those who deny his Holy Name that Jesus taught us to sanctify. 2 Timothy 3:16 All Scripture is inspired of God.  Apparently some people believe only some of the Scriptures were because they're not following all of them.

I don't know if that ''god'' even has a name. I call him the Christian God.

Jehovah is one of them. Adonai, Elohim...I mean...sign...

For the very same reasons, Jesus isn't the name of God's son.

For every reason people try to show Jehovah isn't God's name, that very reason applies to the the name of Jesus.

There isn't a 'J' in Hebrew. Actually there isn't a 'J' in Greek either.

So if the 'J' in Jehovah is wrong, then the 'J' in Jesus is also wrong.
And this would also include the names Joshua, Jehu, Jeremiah.
Then lets not for get our English words, 'Judge', just, journeyed
and so on. 

No one knows the correct pronunciation of YHWH.
True but no one know the correct pronunciation of Jesus' name in Hebrew.

Yet what is the evidence that Jehovah is correct?

Contrary to one popular answer, Tyndale didn't use the name 'Jehovah' nor was it in error.

Tyndale, in his 1530 translation of the first five books of the English Bible, at Exodus 6:3 renders the divine name as Iehovah.

The first copies of the KJV used 'Iehovah' and 'Iesus'

Aleppo Codex is a medieval bound manuscript of the Hebrew Bible. The codex was written in the city of Tiberias in the 10th century C.E uses Yehovah. So does the Leningrad Codex of 1008–1010 C.E.

Which is some 250 years before Raymundus Martini in 1278, and used the name Iehova.

When did the 'I' become a 'J', I haven't been able to find.

Noah Webster's 1828 Dictionary of American English uses the name Jehovah some 39 times, and thus was part of everyday English.

Example: GOD, n. 1. The Supreme Being; Jehovah; the eternal and infinite spirit, the creator, and the sovereign of the universe.

The American Heritage Dictionary (Second College Edition)

Jehovah: “God, esp in Christian translations of the Old Testament.

Brown-Driver-Briggs’ Hebrew Definitions [BDB Definition, c.1906]

H3068 יהוה yehôvâh
Jehovah = “the existing One”
1) the proper name of the one true God
H3091 יהושׁע / יהושׁוּע yehôshûa‛ Joshua or Jehoshua = “Jehovah is salvation”

New American Standard(r) Updated Edition Exhaustive [NASEC, 1998 edition]

H3068 יהוה Yhvh (i.e. יהוה, Yehovah or יהוה, Yahveh) (217d); from H1933b; the proper name of the God of Israel:

G2424 Ἰησοῦς Iēsous; of Heb. or. H3091; Jesus or Joshua,
H3091 יהושׁע / יהושׁוּע yehôshûa‛ Yehoshua (221c); from H3068 [Yehovah or Yahveh] and H3467;

Strong’s Hebrew and Greek Dictionaries [Strong’s, 1890]

H3068 יהוה yehôvâh yeh-ho-vaw' (the) self Existent or eternal; Jehovah, Jewish national name of God: - Jehovah, the Lord.

G2424 Ἰησοῦς Iēsous ee-ay-sooce' Of Hebrew origin [H3091]; Jesus (that is, Jehoshua),
H3091 יהושׁע יהושׁוּע ‛yehôshûa‛ yehôshûa‛ yeh-ho-shoo'-ah, yeh-ho-shoo'-ah Jehovah-saved;

Thayer’s Greek Definitions [c 1889]

G2424 Ἰησοῦς Iēsous Jesus = “Jehovah is salvation”


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Where did the idea that God 'doesn't need' a name?

Today, many pseudo Christians, unwittingly quote Plato when they teach:

“They were strongly influenced by several works of Plato (427- 347) wherein he explained that no name could perfectly designate God,”

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It is the English version of Yahweh formed from the consonants YHWH, Jehovah, and it is God’s name. Jesus is the English word for Yeshua. Since we are not Jews, we use the English names. 
I think it’s interesting that names are very important to God and since we were made in his image, they are very important to us too!  He gave the first human he created a name and gave Adam the opportunity to name all the animals. When Jehovah created the woman, God allowed Adam to name her, he chose Eve. God has named every star and all the angels have names, so I don’t think God would be angry if we called him by his name, Jehovah. It distinguishes him from any other gods, who also have names. The model prayer that Jesus left us glorifies God’s name first (hallowed be thy name, let your name be sanctified), followed by his kingdom and his will. Matthew 6:9-11. Our Heavenly Father set the example!  I think we should follow it!

Because it's a mistaken interpretation of an attempt to vowel point the Tetragrammaton in such a way to avoid blasphemy by actually writing it in a pronounceable way.  You can call God that if you want but it's no more accurate than Odin or Zeus (which are in fact accurate in a sense).  There's nothing special about the name.