Why did Othman Burned the Qur'an which was not agreed by All Muslims?

Because This Qur'an was compiled by Prophet Muhammad peace be upon him Himself.

So it's not allowed for anyone to add or delete in this Divine Book.

Secondly many enemies of Islam deliberately misunderstood the whole issue of burning and compiling on one Dilect.

When Hazrat Othman may (Allaah be pleased with him) took charge of Caliphate in Madeenah Islam began to spread very fast.

Many of these new Muslims were not native Arabic speakers, or they spoke a slightly different Arabic pronunciation from the tribes in Makkah and Madinah.

People began to dispute about which pronunciations were most correct.Caliph Othman bin Affan took charge of ensuring that the recitation of the Quran is a standard pronunciation.

Its not about different copies of the Qur'an but different pronunciation issue which was solved by a very simple and easy way By great caliph Othman bin Affaan ( May Allaah Be Pleased with him)

God took the responsibility of preserving the Divine revelation

“So Allaah revealed

لَا تُحَرِّكْ بِهِ لِسَانَكَ لِتَعْجَلَ بِهِ ?16? إِنَّ عَلَيْنَا جَمْعَهُ وَقُرْآنَهُ ?17?
(Iterpretation of the meaning??)
‘Move not your tongue with it (O Muhammad), to hasten with recitation of it (the Qur’an).Indeed, upon Us is its collection (in your heart) and (to make possible) its recitation

[al-Qiyamah 75:16 – 17]