Because if it were successful it would cost the company a lot of money.
Just threatening to do it might be enough.

Follow me here. The only way we are ever going to get a new contract is when the company finds it in their best interest to sign one. In order for them to get to that point we have to exert economic pressure. We have various legal methods of doing that. Simply living with C2012 and continuing to collect our profit sharing is one way but I doubt it will be enough. Our "ask" has gone up a lot higher since early 2015.
Informational picketing is great too. But it doesn't hurt them enough. It doesn't cost them money.

Its my opinion that we have grown too large for the NMB to ever release us to self help. Its now politically impossible. "Consolidation" means we've seen the last of major airline strikes. The "big 4" airlines are now basically public utilities. We will never be allowed to shut down Delta Air Lines. Therefore Section 6 of the Railway Labor Act is a dead letter. Its useless. The NMB never releases anyone. Negotiations go for years beyond amendable dates.The NMB has made themselves irrelevant. We have to go through the motions to stay legal. But that's all it is. A kabuki dance from the 1960s.

I think we are going to have to escalate in other legal ways.
Bringing or threatening to bring other unions on the property is a surefire way to get management's attention. It will cost them big money and perhaps even more important - they would lose a lot of control over the operation. They cherish that control.
They slashed the non-contract employees' profit sharing by simply issuing a memo. What do you suppose keeping that power is worth to them?