We can only guess, but perhaps they've applied some sort of commonality filter and dropped words that are rarely used? Or perhaps they've simply moved more words from the guess list into the challenge list? (the challenge list was produced from what words one person immediately recognised, after all)

Or, most likely IMO, they were not 100% certain they could legally use the existing word list (now, or for what they plan for the future) or were certain they couldn't without paying for some sort of license, so they produced a new list from scratch from a source they have licensed for their other word games.

There are two lists: guesses you are allowed to make, and possible solutions from the puzzle. From the solutions list they removed some words that had different British and American spellings, as well as some words with negative associations, like "slave."

From the possible guesses list, the words removed appeared to mostly be quite offensive slurs.