If the question is "why would these bishops file suit against Schofield," then the answer would seem to be pretty simple. If they can beat him to the punch and move first, than any move that Bp. S makes to separate from TEC and their heresies might be blocked or delayed by being tied up in ecclesiastical and perhaps civil courts.
If the question is "why haven't they filed suit yet," then it is either because God is bringing these heretics to confusion, or because they are having trouble thinking up reasons for filing suit against Schofield that won't get them into even more trouble with worldwide Anglicanism than they are already in.
TEC would be smart to just let that small portion of their dioceses and parishes leave. They will still control the overwhelming lion's share of property and money. We already know they don't care about how many faithful they have...
I have no background on any of this.
TEC probably fears that allowing the departure of the minority would legitimize the process, thus making it something worthy of reasoned consideration by the cautious faithful remaining within the church. By radicalizing the orthodox and shrouding the actions of the orthodox with the appearance of illegality, TEC is reinforcing it's intimidation.
An "iron vestibule" has decended upon Anglicanism from New Hampshire on the Atlantic to San Francisco on the Pacific!