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Is there a reason why most if not all Android TV box suppliers do not upgrade the Android OS??

Yes. Once they ship a box they appear to stop spending any time or effort developing for it and are already working on the next one. I imagine many of the no-name manufacturers don't see the point in providing upgrades to a device they've already sold and which will generate no income for them.

I suspect your Android phone comes from a well respected brand like Samsung, Sony etc. and may have been bought via a carrier who get continued income from you, so the situation is a bit different.

You'll notice that some companies - nVidia, Wetek, Google etc. DO provide continued support and update for their Android boxes... These are companies that invest in supporting their customers.