It may be due to the fact that they can't control the level of software on your Retail box. You are probably on 14.5.0 by now an update which BT are delaying for some reason known only to themselves.
Why do you think BT are delaying an update - as mentioned a while back the updates branched off and will reconverge at a later date.
We've had a response from the Youview teama and they've confirmed that customers with Retail boxes can’t yet get Extra TV channels although we’re looking to add this option but we don’t have a date yet. The only channels customers can add to a Retail box at present are BT Sport.
"Humax are doing a rolling update to retail customers once this is complete then the attention will turn to boxes made available via BT and Talk Talk. A Humax decision."
Right so YV blame BT
BT blame YV
and you blame Humax.
I would wager that that there are significantly more BT supplied Humax boxes than those sold through the retail channel. Previous software updates have not taken this long to hit BT boxes.