Performance. For example if you are syntax highlighting a programming language with hundreds of keywords, then using a regexp like "(kwd1|kwd2|...|kwdn)" is not very efficient. An optimized regexp can do the same matching much faster.
You would be much better off just generating the DFA of the straightforward regex and minimizing that DFA. This is simpler and faster to generate and faster to execute. Furthermore, in a parser for a programming language you do not want to match some things in one list but not in this other list, you want to match some things in this list and nothing else.
I know this isn't what you're asking, but I imagine in this case it's because that's one of the challenges of regex golf. Matching regex, as short as possible.
