This is because as soon as one ad gets a click, Facebook floods that ad with impressions, and the other ads are ignored. The best way to handle this is to pause all other ads in the campaign, and let the first one run for a day or so. Then pause the first ad and reactivate all the others until one of them pulls ahead. Repeat this process until you have at least three ads with decent CTRs. Then pause everything and let those three run simultaneously. You’ll find that all three will get equal impressions, and you can run a proper split test.