Why is Steamboat Willie still copyrighted?

I'm curious to know, why is it that Steamboat Willie, an animated short film released over 90 years ago, is still under copyright protection? Is it because the laws surrounding copyright have remained unchanged for such a long time? Or is there a specific reason why this particular film has been granted an extended copyright period? I'm particularly interested in understanding the legal basis behind this decision, and how it compares to the typical copyright terms we see for other works of art and entertainment.