I've always wondered why DDG doesn't go fully open source? Their mobile apps and browser extensions are already open source, why not open source the search engi

I've always wondered why DDG doesn't go fully open source? Their mobile apps and browser extensions are already open source, why not open source the search engine? Is it just because they don't want to deal with the extra work (they already shutdown duckduckhack), or is there something else that I am missing?