Why did only Lord Shiva drink the halahala poison from Samudra Manthan? Why didn't Lord Vishnu?

Why did only Lord Shiva drink the halahala poison from Samudra Manthan? Why didn't Lord Vishnu? Only Parameshwara is capable to drink most deadliest Halahal Poison. Srivishnu is actually preserver of universe. Srivishnu is capable to do anything for protection of universe until when They are in his control. when Srivishnu understood himself that which is beyond his control . Then only vishnu take complete help from Parameshwara for protection of universe . All devas of universe including Brahma & vishnu prayed Parameshwara by chanting “Om Namah Shivay” Immediately Parameshwara enter in to the spot took Halahal deadliest poison .all devas are very happy Praised Parameshwara as “Har Har Mahadev” In this way Srivishnu protected the universe by taking complete help from Parameshwara. That's the reason lord Shiva is called Ultimate creator, protector & Distroyer. Lord Shiva is called Neelakanteshwara /vishakanteshwara