simple answer is it doesnt have a high ROI
it is vastly easier to make a game with a replay system in mind then add one after the fact. bottom line is that now this is a project that requires vast amounts of recourses, money, and time, and has no clear impact to player base retention/expansion.
can you complain that this game wasnt launched with a replay system? sure, but in addition to riot wanting this game to be competitive at its core (not content), its release time was perfect for the gap it targeted in the market and they wouldve missed out big if they delayed release because of another big project.
trust me, if this project truly meant the world to enough active players, riot would have had it done already. it is a very vocal minority who care the most about it, as well as the people that support those few.
there are a lot of people smarter than you and me working at riot that decided there was no rush to get this completed soon, that tells me all i need to know.