Simple. Because it is not in the personal interests of the Senator Judges to see a Supreme Court Chief Justice fall under the weight of a "scandal" wrought by secret dollar accounts made public by a Malacañang juggernaught intent on bulldozing through civil liberty safeguards and due process.
Once Malacañang is through with its enemies in the Supreme Court and a multitude of unlawful precedents had been set in the process of this purge, it is likely that the Senate will be next. The House of Representatives having already earned their brownie points from a "furious" President will likely be spared his Gestapo.
Acquit Corona and the Senate will have a Supreme Court that owes them a debt of gratitude. Everyone who has interests vested in the status quo (guys and gals with millions to hide) wins. And that means the country's Senators will be able to sleep well in their comfy airconditioned bedrooms at night.
It's simple, really™ -- though not for the small-minded.
Kabahan na kayo, mga Senador. Allowing Malacañang to arm itself with precedent unlawful evidence gathering practices that it had applied with impunity under the watch of your fine court will ultimately be the undoing of your golden nest eggs.