The Front Page (1931) & His Girl Friday (1940)

Sometimes, those things just slip through the cracks; clerical oversights or whatever. Even the prestigious MGM has allowed copyrights to lapse. Till the Clouds Roll By and Royal Wedding were examples of major, star-studded musicals that fell into the public domain. 

But as spiderwort pointed out above in the case of It's A Wonderful Life,* it can actually benefit a film's popularity and longevity. In addition to broadcast availability, the advent of home video allowed anyone with sufficient funds to master and duplicate even the most battered 16mm print of a PD title, and market it at cut-rate prices. Racks at supermarkets and drugstores used to be filled with them.

*After the war, director Frank Capra had formed his own independent production company, Liberty Films, under the banner of which he directed only IAWL and State Of the Union (which, if memory serves, also fell into PD) before the company went defunct, and this was likely the reason copyright(s) lapsed. 

For some years, the original production entities would neglect such titles but, as home video markets broadened and higher-resolution video media gained market penetration, they realized there was money to be made by those retaining original negatives or other high-quality preprint elements, and that consumers would willingly pay premium prices for the best image and sound quality.