Background: Sheikh Mujibur Rahman (Bengali: shekh mujibur rhmaan); Bengali pronunciation: [Shekh Mujibur Rohman]; (17 March 1920 - 15 August 1975), shortened as Sheikh Mujib or just Mujib, was a Bengali politician and statesman. He is the founding father of the People's Republic of Bangladesh. He served as the first President of Bangladesh and later Prime Minister of Bangladesh from March 1971 until his assassination in August 1975. He is considered to be the driving force behind the independence of Bangladesh.
Context: Mujib was arrested by the army and after two years in jail, an official sedition trial in a military court opened. Widely known as the Agartala Conspiracy Case, Mujib and 34 Bengali military officers were accused by the government of colluding with Indian government agents in a scheme to divide Pakistan and threaten its unity, order and national security. The plot was alleged to have been planned in the city of Agartala, in the Indian state of Tripura. The outcry and unrest over Mujib's arrest and the charge of sedition against him destabilised East Pakistan amidst large protests and strikes. Various Bengali political and student groups added demands to address the issues of students, workers and the poor, forming a larger "11-point plan." The government caved to the mounting pressure, dropped the charges on February 22, 1969 and unconditionally released Mujib the following day. He returned to East Pakistan as a public hero. He was given a mass reception on February 23, at Racecourse ground and conferred with the title Bangabandhu, meaning Friend of the Bengal.  Joining an all-parties conference convened by Ayub Khan in 1969, Mujib demanded the acceptance of his six points and the demands of other political parties and walked out following its rejection. On 5 December 1969 Mujib made a declaration at a public meeting held to observe the death anniversary of Suhrawardy that henceforth East Pakistan would be called "Bangladesh":  There was a time when all efforts were made to erase the word "Bangla" from this land and its map. The existence of the word "Bangla" was found nowhere except in the term Bay of Bengal. I on behalf of Pakistan announce today that this land will be called "Bangladesh" instead of East Pakistan.  Mujib's declaration heightened tensions across the country. The West Pakistani politicians and the military began to see him as a separatist leader. His assertion of Bengali cultural and ethnic identity also re-defined the debate over regional autonomy. Many scholars and observers believed the Bengali agitation emphasised the rejection of the Two-Nation Theory - the case upon which Pakistan had been created - by asserting the Ethnocultural identity of Bengalis as a nation. Mujib was able to galvanize support throughout East Pakistan, which was home to a majority of the national population, thus making him one of the most powerful political figures in the Indian subcontinent. It was following his 6-point plan that Mujib was increasingly referred to by his supporters as "Bangabandhu" (literally meaning "Friend of Bengal" in Bengali).
Question: why did they want to erase it?. Whats the answer?
The existence of the word "Bangla" was found nowhere except in the term Bay of Bengal.