In a May 1993 Word Ways article, Donald Knuth says that the reason his computer found so many 5x5x5 word cubes is that so many five-letter words can be tweaked" into others. Each tweak or combination of tweaks can produce a new cube, only slightly different from the old one.
"Significantly-Different Word Squares,"
Word Ways: Vol. 26
, Article 14.
Available at: http://digitalcommons.butler.edu/wordways/vol26/iss4/14