Because he's still protecting Sherlock.
Mycroft has to reveal/confirm the existence of Eurus because he's trapped into admitting something Sherlock has worked out anyway.
But that doesn't mean he still wants Sherlock to know everything she's done.
Mycroft to Sherlock...
"Memories can resurface; wounds can re-open. The roads we walk have demons beneath ... and yours have been waiting for a very long time. I never bullied you. I used – at discrete intervals – potential trigger words to update myself as to your mental condition. I was looking after you."
Remember, Mycroft doesn't know that Eurus has control of Sherrinford until after they are nearly killed by the 'patience grenade' and even once they are on Sherrinford and trapped in the 'game' he doesn't know Eurus' final intention...just that she's torturing all three of them...albeit, mostly Sherlock.
He might consider that the 'big reveal' is her plan but he's still trying to prevent Sherlock from harm as he's always done.
For all he knows they might get out of it without him having to tell Sherlock at all.