It happening every 18 months doesn’t help if it’s not happening every 18 months over a zoo or other location that studying animals is easy. You can browse future solar eclipses here and see how many are over the arctic/antarcitc, open ocean, a tiny portion of the tip of a continent, etc.
It’s also something that lasts for only a few minutes when it does occur, so it’s going to take a many eclipses to collect large amounts of data.