This isn’t truly a historical trend, so much as a shortcut by artists.

Drawing hands are some of the most difficult pieces of anatomy to get right. Even experienced artists will make goofy looking hands. Parts to get wrong include: Movement of fingers Proportion of finger joints Proportion of fingers to each other Proportion of hand size with body Intricacies such as hair, creases and nails

When hand drawing every frame such as the old Micky mouse cartoons, the process is simplified when there are less fingers. Depending on the character and style of the animation, an artist may get away with 3 fingers and a thumb (thumb may be omitted in many scenes it is not necessary) with out the figure looking too odd. This is easier with anthropomorphic characters as the entire character is already more imaginative that the number of fingers doesn’t strike the viewer as specifically odd.

With more detail focused, human characters in animation you may see 4 fingers and more occurrences of thumbs when not in use. This is more common in the realistic anime style as opposed to classic American cartoon animation.

Now most cartoons are drawn and entered into a computer and manipulated to develop an animation. This makes it more possible to spend the time to get the hands looking right at the initial stage and then the computer uses the template for the rest of the character manipulation. even so, reducing fingers on lower quality designs saves a lot of time and therefore money, and if the design allows for lower detail resolution it doesn’t look noticeably off.

TLDR: hands are real hard and expensive to draw well, sometimes less is more.

Source: discussions with my SO who is a freelance artist, BA in fine arts, and is self teaching animation.

Also the documentary on how south park is produced “six days to air”.