Volunteers - or even levies for colonial service - are not
Volunteers - or even levies for colonial service or even rear-area services of supply tasks - are not the same as conscripts for combat service in the trenches in 1917-19, as presumably should be clear from the context.
If not, of course, it raises the obvious question of why the Indian Army corps that went to France in 1914 was withdrawn, much less not reinforced and supplemented to the point where the British high command would not have needed to institute conscription in the UK, much consider it in Ireland, Canada, etc despite the rather obvious historical opposition to such ideas.
Much less the issues of conscription for overseas service in the white Dominions; you may also consider how (for example) the (white) South Africans reacted to the ideas of British recruiting of volunteers for combat units in the then-British territories in Southern Africa.
The realities of early Twentieth Century racism and the color bar/color line/etc are hardly unknown historically, across the West, including the US, UK, the empire(s) etc. No Western nation mobilized its non-white population in the way described above at this time.
Finally, the reasons why are quite clear - as a fairly well known Briton and observer of the British Empire once wrote "how long can we keep kidding these people?"...
Best,