And who are we to SPECULATE on the whys and whos and whatnot ? Why was Guru nanak ji born in 1469..and NOT 1969 ?? why in Pakistan and NOT in Montreal ? Why Gurus wrote bani on Paper with INK and not on clay tablets/Gold tablets/marble stones ? Why Gurus never commissioned the making of huge statues like the ones the talibans destroyed in Afghanistan....a million why and whynots can be asked....so easily.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/37825-why-guru-gobind-singh-jis-bani.html
1. The FACTS: Guru Nanak Ji collected the BANI that passed HIS TEST when He went on hsi worldwide journeys called Udasis. That is HOW the Bani of Kabor ravidass Ramanand, Namdev ec got its place in the Sri Guru Granth Sahib Ji.
2. Guru nanak Ji wrote His own Gurbani....as and when it came - and SANG it to the congregations daily till it was on the lips of many who daily visited the congregations as Bhai Gurdass Ji states in his vaars.
3. Guru nanak Ji passed on THIS GURBANI as the "Mark/Title of Succession" to the successor Guru. NOTHING else could mark the next GURU except GURBANI GYAAN. Same happened with each SUCCESSION.
4. Guru Amardass Ji first thought of constructing a Central Place of Sikhism...BUT Clealry explained that the REAL CENTRAL ABODE of the CREATOR is the HARMANDAR - OUR OWN PERSON...THE BODY that we are born with. Sure enough even with this CLARITY..when the BUILDING came to be constructed by Guru Arjun Ji sahib....much later this STONE BUILDING came to be.."portayed" as the ABODE of the CREATOR---HARMANDAR" even though the SPELLING of the word clearly shows a SIHAREE the Pronounciation is DELIBERATELY misinterpreted as BIHAREE to TWIST the HAR into HARI !! Genuine Sikhs find it hard to explain HOW..the Creator could be residing in this Harmandir when Guru nanak ji went all the way to Mecca to teach the Mulsims that the CREATOR cannot be residing in mecca !!
5. Guru Arjun Ji compiled the completed AUTHENTIC GURBANI into the AAD GRANTH....and SEALED the GRANTH via His Personal MOHR or SEAL called Mundawwnni Mahalla Panjavaan. (SLOK of Thanksgiving Tera Kita jatoh nahi is according to the symettry of the Granth because it also BEGINS with a SLOK..aad sach jugaad sach !!).
6. Guru hargobind, Guru har rai and Guru harkishan Ji DID NOT WRITE nay GURBANI and the AAD GRANTH remained as it was. ( Lots of SPURIOUS 'gurbani" was however penned by the DHIRMALL GROUP opposed to the GURUS under Mahalla Chevaan, Mahalla satvaan )
9. Guru TEG BAHADUR ji wrote LOTS of GURBANI....even ADDING a NEW Raag that ONLY Guru teg bahdur Ji used in Sri Guru Granth Sahib Ji. Now heres a dispute of sorts. Many beleive that Guru Gobind Singh ji ADDED the Gurbani of Guru teg bahdar Ji at a place called DAMDAMA SAHIB near Paonta sahib in the mountains of Anandpur...and such COMPLETED BIRS of AAD GRANTH complete as we know it today as Sri Guru Granth Sahib Ji were COPIED and distributed in Guru Gobind Singh Jis presence where 52 kavis schoalrs writers were present at His darbar in Anandpur. The more popular story is that the Damdam sahib is the one in BHATINDA. The Scholars need to go further into this aspect of Sikh history because lots of Birs exist prior to Bhatinda Damdama sahib incident but they contain the Gurbani of Guru teg bahadur ji in full.
7. The FINAL indisputable FACT is that Guru Gobind Singh Ji had ALL the time in the world to ADD His bani (IF he indeed wrote any....and Hes still our GURU just as Guru hargobind ji or Guru har rai ji and Guru harkishan Ji who also NEVER wrote any Gurbani ...THIS REFUTES the totally spurious argument/logic that many pro-dg use to blackmail Sikhs..that How can we say " Sri Guru Granth Sahib Ji..is...Dassan Guruan dee bani when we leave out the alleged writings of Guru gobind Singh Ji.....The GURUSHIP of Sri Guru Granth Sahib Ji is based on the FACT that the Tenth nanak bestowed the SUCCESSION- GURGADEE on Sri Guru Granth Sahib Ji as the Next and Last GURU after HIm.....the Gurgadee is NOT Based on who wrote or didnt wrte what ?? IS Bhai lehnaa Jis SUCCESSION as GURU ANGAD based on the fact that Guru NANAK JI bestowed Gurgadee on Him or on whether Bhai lehnna Ji wrote Gurbani ??).
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=37825
8 OUR GURU NOW..is One and ONLY Sri Guru Granth Sahib Ji. ONLY the GURU speaks GURBANI. PERIOD.