Professional baseball already existed. What it did not have was a system sturdy enough to make anybody trust next month's schedule.
Players jumped to whichever club paid more, sometimes in the middle of a season. Clubs ran out of money and disappeared with games left to play and wages still owed. Two teams could split one city's audience. Gambling rumors hung over results.
The National League did not invent professional baseball. Its narrower achievement was making club-owner control durable.