Disney animators literally wrote the book on creating incredibly lifelike animation and characters that come to life on the screen, it's called The Illusion Of Life by Frank Thomas and Ollie Johnston.
And in the beginning Disney artists and animators came up with all kinds of clever yet time-consuming ways to make their animation look better than any of the stuff the other studios were putting out, setting the bar high for this relatively new artform.
But hard times hit Disney's feature animation division in the late-60s, and budget cuts forced the animators to find new ways to keep a cohesive look yet cut down on the cost of the animation process.
So they decided to recycle old sequences with similar characters in similar situations to make something old new again in half the time and for far less money, the same reason they introduced 3D backgrounds in films like Beauty and the Beast and Aladdin.
Now Disney animator Wolfgang Reitherman claims they didn't start recycling clips to save time or money but because they "wanted to stick to what they knew would do well", but I'm sure it didn't hurt that they were saving money as well.