Because Dark Law is way too powerful for Duel Links right now. If you summon him alongside a bunch of defensive traps you pretty much shut down most meta.
If you can't see for yourself that Dark Law is far stronger then Necro valley I suggest you to go in a TCG local tournament and ask around or watch some videos on YouTube
Dark Law won’t be as broken in Duel Links. DL doesn’t nearly have as many graveyard reliant decks as the TCG. Some decks like spell books, Koa, and vampires need to be knocked down anyways.
I don't know why Konami and many stupid yugioh fans wants so many hero supports even when hero's are not part of main series but no dark magician supports been added yet even the green faced dark magician animation sucks finally yami yugi got master of Magician but still dm needs box and structure deck like blue eyes got