BT's channels are delivered by IP. Adding a second 4K channel would add additional line speed restrictions (possibility of watching 1 x 4K channel and recording another). 4K is transmitted at roughly 30Mbps so having capability for 2 would mean needing a line speed in excess of 60Mbps which a lot of people signed up to the service would not get.
The only options would be to transmit either on BT's 4K channel (Not sure if that is a sticking point) or have 2 part time channels (again probably not feasable).
BT could ask Eurosport to provide UHD catch-up content but this isn't really the idea. If they added a player they could deliver it through that (ala Netflix) but a bit late in the day to do that.
I presume SKY will deliver broadcast via thier satelite bandwidth, not sure about Virgin but they don't have a dedicated 4K channel so it will only be one and they tend to have a faster BB service than BT customers.
Would be interesting to know the actual reason but I don't suppose anyone will ever tell us that (commercial confidence and all that).
I have tested it myself but even a quick google comes up with :-
'Ultra HD content looks brilliant, helped by the high bit rate of about 29 Mbps currently being used, which is nearly double that used by Netflix and Amazon for their Ultra HD services.'
BT could offer extra UHD channels, but make it clear that only one UHD channel is available at any time (can be used to view live, or record live). I would welcome such a service. May have to consider switching back to Sky and cancelling BT TV. My move from Sky was when Sky warned that EuroSport may be deleted fro Sky.