There are two reasons that outside planes land on the itty-bitty Island of Baltra. One is the U.S. military built a landing field on it during WWII because it was the flattest island and easiest to work with. That making the cheapest way to have an airport: just maintain and upgrade the one someone left you. The other reason is Baltra is pretty much isolated from the other islands. There are other islands that are even more isolated from the inhabited islands, but they are not close enough to them that it would be easy to get workers back and forth to the airport. And as we found out before we even landed, the goal of the protectors of the Galapagos is to keep exposure of any outside things from further contaminating the ecological balance of the islands. So Baltra acts as barrier island to the main populations.
It starts on the plane. As we approached the landing field the steward opened every overhead bin then walked the full length of the plane spraying the carry on baggage. I know they were spraying for unwelcome bug hitch-hikers, but some how I felt as though they thought we all had cooties.