Because he is under a lot of pressure not to be too daring - not just on this but on many issues. And from many sources - including the people running his office, who are generally to his right and don't have his campaigning background. Some of them are Stalinists who positively welcome the purging of Trots.
Corbyn argued early on, on national TV, that it was fine for revolutionary socialists to be in the Labour Party, but as far as I know has said nothing even about the purge in general since then. John McDonnell has made a couple of public statements about the issue but done little more than that so far.
Until now, part of the pressure was feeling boxed into a corner. Now is the time for the left to urge them to be bolder, on this and many issues!