Because when I try to install, it tries to install for the latest version I have only. I have two versions: 1.8 and 1.14, right now optifine is only installed in 1.14, because I can't find the 1.8 specific jar file. This is using the new launcher, meaning there's an "Installations" tab, etc. (When I open folder for the 1.8 installation and try to install 1.8 optifine to the jars there, it doesn't install (without errors, and says installation complete, but booting it does nothing, and there's no 1.8 optifine installation), or says it's an invalid install.)
Example invalid install error:
yes, I've run 1.8.9 at least 4 times. Even connected to Hypixel, installed the 1.8 Dokucraft / gerudoku resource pack, etc.