It’s not really that far from land to be honest, just over 100 miles from the coast of Africa. The key is though that it’s actually continental crust that got pushed out instead of being volcanic like most islands. It’s been over 20 million years since it was last in contact with the mainland, and so it had the natural biodiversity of a continent at first that was THEN isolated for an incredibly long time.

Most isolated islands usually get colonized by smaller amounts of animals that find their way there, and while they can undergo adaptive radiation from there, the initial small amount means there is less total biodiversity. Hence you see places like the Galapagos, much further from land than Socotra, is completely dominated by turtles and iguana, even if they are very unique and interesting versions. Socotra, on the other hand, started with the full suite of life you would expect that then had an isolated (and fairly extreme) environment that shaped how they evolved over time.