Losing digits seems relatively easy - that's where hooves come from (horses have one toe left, goats two). It's gaining digits that seems very hard. It's possible that some configuration of six fingers could be better, but we probably can't get from our local optimum to that without going through major disadvantages. Apparently five has been the maximum number for most creatures for 420 million years: https://www.reddit.com/r/askscience/comments/1org2e/why_do_m...
