The official reason given, in the words of the UK Foreign Office, is "The referendum may increase instability in the region, at a time when the main attention should be paid to the victory over ISIS."
But there will always be risks with doing anything productive. And there will always be strong opposition to any national movement.
The UN Charter speaks about the importance of “equal rights and self-determination of peoples” and later declarations elaborated on it:
By virtue of the principle of equal rights and self-determination of peoples enshrined in the Charter of the United Nations, all peoples have the right freely to determine, without external interference, their political status and to pursue their economic, social and cultural development, and every State has the duty to respect this right in accordance with the provisions of the Charter.The UN doesn't say that people's right to self-determination should be pushed off indefinitely if it might upset other people.
Every State has the duty to promote, through joint and separate action, realization of the principle of equal rights and self-determination of peoples, in accordance with the provisions of the Charter, and to render assistance to the United Nations in carrying out the responsibilities entrusted to it by the Charter regarding the implementation of the principle, in order:
To promote friendly relations and co-operation among States; andand bearing in mind that subjection of peoples to alien subjugation, domination and exploitation constitutes a violation of the principle, as well as a denial of fundamental human rights, and is contrary to the Charter.
To bring a speedy end to colonialism, having due regard to the freely expressed will of the peoples concerned;
Every State has the duty to promote through joint and separate action universal respect for and observance of human rights and fundamental freedoms in accordance with the Charter.
The establishment of a sovereign and independent State, the free association or integration with an independent State or the emergence into any other political status freely determined by a people constitute modes of implementing the right of self-determination by that people.
The supposedly enlightened and liberal world suddenly thinks that these words no longer apply - when it comes to Kurds.
Why?
The real reason is that the world regards the Kurds the way it regards Israeli Jews. It knows that those who oppose both Kurdish and Jewish nationalists are irrational, terror supporting states who can threaten the West with terrorism. And the Kurdish and Jewish nationalists (except for the PKK in Turkey) are not going to start setting off bombs in Western cities.
That's really it. Terror has become the most successful political tool on the planet, because it causes Western nations with supposedly liberal values to suddenly throw out all their principles - and justify that hypocrisy after the fact. An irrational, violent actor gets its way while the rational ones who seek peace but true justice are asked, sometimes nicely and sometimes not so nicely, to stop being so demanding.
We've seen this hypocrisy many times before.
Maybe it is time to repeal the UN Charter, or to modify it to say "all of these principles only apply when it doesn't inconvenience anyone too much."