Why will minecraft 1.5.2 (java) work but 1.12.2 (also java) not even open? Is there a way to fix this?

Because there a different versions of Java, and 1.12.2 might require a newer version I guess.

On the support page it says:
Starting from Minecraft 1.12, Java 8 will be required to run Minecraft. If you don't know whether you have Java 8, don't worry - our installers supply Minecraft with its own version of Java by default.