Unless I’ve missed something, Stop the War Coalition hasn’t changed. What has changed is that Stop the War Coalition is suddenly getting scrutiny due an organisation heavily influenced by the leader of the UK’s second largest party. And what that scrutiny reveals makes it hard for anyone aspiring to mainstream politics to be associated with the thing they helped to shape. So they edge away, citing pressures of time &c.
Sunlight is the best disinfectant.
And whatever the reasons, it is the right thing to do.