Amir Abdulazeez ‘s answer puts it perfectly, hardly a way I can out do that, but I will attempt to elucidate on them in four salient points
Lack of a united front
Either MASSOB or IPOB fail to offer a united front with which to present their case in a way deemed legal, within the constitution of the country. Inciting hate and violence, which essentially rally a hostile situation, through a a radio station in London is hardly a decent direction for a potential neighbour.
The recent Catalan referendum is such a prime example of one path which Eastern Nigeria would have pressured the government to act. By conducting a howbeit, illegal vote count, they would have effectively placed a number on the amount wanting separation.
But until the IPOB provides, that majority indeed wants separation The Government’s stance would still remains; they are viewed as a rebels to the peace of the country.
Ambiguity of claim
The Biafran prospective state line worries everyone. The last attempt at Independence had most of the now South-South part of the country lumped together as Eastern Region. But recently, that water has become a little more murky.
The bulk of the then Eastern Region have followed through with an organic partitioning , to create more states for minorities and ethnic groups — not too dissimilar from the Igbos. These parts, The now new separatists still claims even with explicit rejection frm those regions.
The Nigerian Government is hardly going to hand over the oil rich regions and it’s non agreeing citizens anytime soon ( although they did that with Bakassi but that is for another day ) a situation which IPOD has little plan over.
The present situation with Biafra in Red
This leaves Biafra effectively landlocked country —- if granted independent— and an enclave within the Nigerian state. A prospect most Igbo elites aren’t too delighted to carry on with, given that they could be denied the greater market of Nigeria and bottled with levies and import duties they have enjoyed as part of the Nigeria state.
The Biafra war and men who fought it
The vanguard of the winning Nigeria side in the Biafran war, transitioned into government with some now controlling the country in one faction or another.
Given that they were around to receive the then surrender of the breakaway region and the Vice president of Biafra in his speech declared that Biafra ceases to exist. They — the former generals and now government— view the current agitation with disdain, and as a sign of betrayal.
Even more so, the exiled leader of Biafra Dr Ojukwu was pardoned on the grounds of those terms, even allowed to compete in the presidential election of the country in a bit to smooth thing over.
Limited understanding of the geopolitical situation
Nigeria is a power struggle between the Southern and Northern protectorates (colonial terms) . The South subdivides into Eastern and Western region with the Niger delta, a cacophony of smaller ethnicities groups as south south.
The North partitions into, Core North, with the middle belt as another area with large minority. But any true Nigeria would tell you, the real power lies with the major ethnic groups which the Igbo is part.
Instead of a dual power structure, Nigeria is essentially a Triumvirate between Yoruba, Hausa ( sometimes lumped with Fulani) and Igbo with other groups stacking neatly behind.
To gain any significant traction, they have to appeal with the other major groups with reasons to support their independence which the Igbo are yet to accomplish.
Then again there is the old nemesis precedence. Nigeria isn’t currently the happiest place , and The Government, guessing correctly understands that handling Biafara a hall pass would end Nigeria as a state and institution.
Other regions would want to follow suit. And this would be a disaster. It would break the country into small states, demolish the relevance of the country on the continent, and open a chance at going again at each other in a bid to settling old scores during the civil war. This, nobody wants.