Hi there! Here's me again!
So in generation 7, Kyreum-B's 170 attack stat was useless as KyokaSuigetsu says, due to the fact that the psychical ice type move that Kyreum-B can learn was Freeze Shock, and has to use two turn (Only if you has Power Herb) to complete. In this case, Kyreum-B must has to be a mixed one instead of just a physical one. White Kyreum was much more better seems that it got a 170 SpA and a strong special Movepool.
Here's what Smogon says in Gen 7:
Kyurem-B lacks a spammable physical Ice-type STAB move, making the one-use Subzero Slammer its only option to use its base 170 Attack stat. In spite of Kyurem-B's base 95 Speed allowing it to outspeed key targets such as Heatran, Tapu Fini, Excadrill, and Gyarados, Pokemon like Mega Medicham, Kartana, and Volcarona can still take advantage of the fact that Kyurem-B's Ice typing gives it many weaknesses and outspeed and either heavily damage or OHKO it. Furthermore, Kyurem-B's Stealth Rock weakness and vulnerability to burn, Toxic poison, and paralysis make it easier for it to get worn down throughout a match.
So thats why its in OU instead of Uber in gen 7.
For the second Question, Kyreum-Black was now Uber was because of the new move that was given to Kyreum-Black-- Icicle Spear, which coud be up to 125 power which give Kyreum Black goes to Uber. And also, because of something, in the SS dex most of it's check and counters such as Heatran, Mega scizor, Kartana, etc., etc., was not allowed in Generation 8 due to dynamax makes Megas and Z power becoming banned.
Hope it helps!