why can't I open my old iPhoto

Because iPhoto was discontinued 6 years ago. It is no longer developed or sold. Now it's so old tat it won't even run on recent OS versions. But all is not lost: It was replaced by the more powerful (and free) Photos.app that comes with the OS. You can migrate to that app very simply:

Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point at your iPhoto Library. It will do the rest.

If you prefer not to use Photos there are several 3rd party apps that you can use. Which to choose depends on the kind of photographer you are and your budget.