Before we begin, I'm not writing about a specific champion.
When a champion is being released or getting reworked you can instantly pick that champion and go play with it on a ranked game. Since it's a **_new or recently reworked champion_** it's not yet tested by the community and it might be unbalanced or have other issues. Wouldn't be better if for the first month you could play with it only on casual games before being able to pick it on a ranked game? In this way, if the champion ends up having balance or other issues the community will test it first, send feedback to the developers and the issues will be fixed before being able to ruin any ranked games which must be strictly fair and balanced.