I can understand people saying they want sandbox in order to learn mid-late game knowledge / team fighting, buildpathing, maybe avoiding and hitting skillshots, however saying that you need it to practice CS or how to flash walls is absurd, to be completely honest.

The majority of players have been playing for well over a year, and if you haven't been able to CS decently, then you most likely either won't, or will take you a long time to do so.

Sandbox in order to practice CS and flashing would really only help newer players, unless you, as a "veteran" spent a big amount of hours, weeks and weeks practicing it on sandbox (which, to be fair, you could be practicing CS on custom games). Expecting sandbox to show results on yourself as a player without commitment won't happen.

This is why Riot most likely doesn't really take it as their priority. Most players are people who play it casually. Those won't bother playing on the sandbox because they only play for fun, and aren't there simply to practice stuff.

Then you have the high elo players (except for pro players) who won't really play it because they're already good enough, and really do not need to improve much, as they've most likely hit their peak.

Bronze-Platinum players could potentially improve by using the sandbox, but I think people are overrating the possible outcome a bit.

Sandbox will not drastically change the way you play game. It will not make you a better player easily. You will still have to put hours and hours, weeks and weeks of practice in order to either you understand it, or your body being used to it so that you don't even need to think about it.

I honestly find it hard to believe that even 10% would spend more than 30 minutes a day CSing for weeks, simply to get better at CSing.

Sandbox will ultimately help pro players the most, and most likely have medium to little improvement on the average player, unless he's a relatively new player.