My guess is a licensing issue with some of the clips within or they're infringing on BT Sport's rights as the UK broadcaster. I watched it before they pulled it. Pretty good, but far too emotive for a UK audience. The BT no filter boxing is a better fit for this market.
If its now only available in UK via the BT sport website that may suggest Showtime sticking it on youtube with no territory restrictions infringed BT Sports rights, so instead they are hosting it on their site and can drive some additional traffic and ad revenue from it.
It's listed on BT sport to be aired on Monday night. Seen an independent review saying it was the best ever all access episode. I look forward to seeing it.