Becky Sue Epstein explains how champagne became a New Year's standard:
[As the marketing of the beverage first expanded beyond the courts of France and England], royal favor made champagne an easy sell to the nobility. But with the rise of industrialization in the 19th century, the nobles were no longer guaranteed to be the wealthiest consumers. Champagne producers dangled their products in front of the newly rich merchant class: an aspirational beverage. Of course, these new customers couldn’t afford to drink champagne every day, but they could afford it on special occasions. Soon they began ordering it for all celebrations. Champagne became de rigueur at festivities from weddings to ship christenings — to ringing in the New Year.
Meanwhile, Reema Khrais recounts various theories for why we clink glasses:
[T]he clinking of glasses? [Paul Dickson, author of Toasts: Over 1,500 of the Best Toasts, Sentiments, Blessings and Graces] says that toasting flair didn't popularize until the early days of Christianity. Many believed the bell-like noise would drive off the devil — which was most dangerous in times of drinking and reveling. But that's just one theory. Another legend contends that by adding the clink, toasters could get the greatest pleasure from a drink, Dickson says. Before the clink, toasts only satisfied four of the five senses.