Because the film makers wanted to show the sons of the people that would watch at that time, not for future documentaries.
My Dad was a Gunner and the next squad to his was from (I Think don't hold me to this) Bechuanaland, and when the Pathe news crew came round to film pulled the Lads from Bechuanaland off the gun turrets and put some English guys on instead, with the words, the Mothers want to see their sons not some ?????? and good racist comment followed.
Dad still wasn't filmed, but hey Pathe had the footage for the Mothers back in Blighty.
There wasn't the Black film crews going round, so they didn't get filmed.
In WW1 there were many soldiers from India, but you will be hard pressed to find any references.
Likewise the Ascaris who were the East African rifles, and only a few of the Officers were white fought in Burma, but again you will be hard pressed to find any references on t'interweb to their even being in existence, not because of any deliberate or even accidental reason, but purely because they didn't record their own exploits.
The other point is not everything that was recorded has been released, and that is down to individuals, back with my Dad, we cannot find any mention anywhere of his Battalion the first Heavy ack ack on t'interweb, unless or until someone releases the info onto the web it is invisible.
The majority of stuff was filmed by BBC or Pathe News, and the explanation is above, they wanted to show the sons out there on the front, they were the ones getting killed, that the public in Blighty were interested in.