Because it's short sighted and ultimately detrimental to the player, the game, and people's perception of Riot.
Lets look at why people want the old HUD. They're unfamiliar with the new HUD, and haven't spent enough time with it to get adjusted. There are some things which could be improved on it, but that's not worth creating an option to remove it all together. But rather than them spend the time to get to know the new HUD, they want developers (who would be focused on improving the new HUD) to go and add a way to swap to the old HUD.
The problem is that won't be the end of it. Eventually, inevitably, Riot will come out with something that requires custom HUD work to be done. Maybe it's a champion who's abilities all level up with their level, instead of putting points into them. Are you going to tell players they can't play that champion on the old HUD? Maybe it's another Ultimate skin with a custom button. Are you going to tell players they can't use a skin on the old HUD? You're requiring more maintenance of something that's discontinued, and more testing and developer time too. You're potentially limiting what can be added to the game, because you want to keep the option of an old HUD.
The old HUD has a feature, custom chat location that saved positions between games. That feature broke when Azir was released, and was never fixed because of how relatively unused it was. As I recall, there wasn't any HUD related changes then, it just broke for no apparent reason. An option to go to the old HUD would inevitably be very under-tested at Riot as it's a discontinued feature, so we can assume the HUD eventually starts to fall apart in similar places. Maybe the old HUD stops being able to see if Allies have their ultimates up, or item positions don't match up in the HUD any more, any number of things could break. This makes the players who opted to use the old HUD upset that it's broken. They take to the boards, Reddit, player support, etc. to complain about it, and Riot tells them exactly what they told them when they released the feature, that the old HUD is no longer supported. The player isn't satisfied, and Riot is spending time and money telling them it's not supported.
Then, 2-3 years from now, the HUD changes again. What do they do then? Do they keep three HUD options around? No, that's too much to even try and manage. So they remove the old HUD then, and you're in this same boat of having adjust to a newer HUD. You've postponed the inevitable, and created more work and troubles for everyone along the way, just so you can maintain what you're use to a little longer.
That's why I can't support an option to keep the old HUD around as an option, and why I feel constructive feedback on what to change in the new HUD is far more productive than asking for an old HUD option.
