To answer this question I think it's important to first talk about the Inuvialuit's own land claims agreement, which concluded in 1984. Negotiations for this claim were largely instigated due to a proposal to build a pipeline down the Mackenzie Valley. The proposal was largely objected to by Indigenous peoples, including the Inuvialuit, because they wanted to benefit from the resources being drawn from their ancestral lands. In order for the pipeline project to be undertaken with the specific goal of benefiting Indigenous groups, it was crucial that said Indigenous groups have their sub-surface land rights be recognized by the Canadian government, and these sub-surface rights did in fact make their way into the final agreement between the Inuvialuit and the government.

Now consider both the Canadian government's demonstrated willingness to negotiate land claims as well as the chronology of the Nunavut Land Claim specifically. The Government of Canada for many years used the Indian Act to impede or outright ban land claims, and even in recent times has hardly been eager to enter into negotiations with Indigenous groups. Even when dealing with untreatied land, the process is slow and concessions are hard-won (the Nisga'a agreement is a good example of this). It has been even harder for Indigenous groups who signed treaties with the crown, the Hudson's Bay Company, or one of Canada's/British North America's colonial governments to renegotiate those treaties. When nations that ceded their lands during the 1700's or 1800's through arguably quite deceptive negotiations processes cannot get the Canadian government to return to the treaty table, what is the likelihood that the Inuvialuit could reenter negotiations less than a decade after their land claim was finalized? From the Government's point of view, the terms of their land cession were already agreed upon and they no longer had the right to make land claims.

You can see that the Inuit negotiators responsible for the Nunavut Land Claim were aware of this when you look at the evolution of their demands and tactics. They knew how crucial it was to get the land claim right, because renegotiation was by no means guaranteed, or even likely, to be granted. Taking this into account, they incorporated elements of recent negotiations, significantly those of the Cree and Inuit of Northern Quebec (resolved 1975) and the aforementioned negotiations with the Inuvialuit. Inuit negotiators demanded things like a hunters' support program and subsurface land rights because they knew that the Canadian Government viewed land claims as one-time real estate deals, not long-lasting partnerships.

To summarize, all the sources I'm familiar with point to timing and the broader context of the Canadian government's attitude towards land claims as the reason why the Nunavut Land Claims process did not involve more Inuit populations (if you would like to read something on the topic, my best recommendation is "Nunavut: Inuit Regain Control of Their Land and Their Lives"--I believe there is a pdf of it floating around on google). However, inter-territorial/provincial politics (particularly between the Northwest Territories and the then-proposed territory of Nunavut) certainly played a role, as did simple geography and the nature of population distribution in the North, but my knowledge of these areas is far less extensive.