There were options they could choose such as staying in other countries that they believed no World Class Monster stayed inside, such as Bolivia, Colombia, and Venezuela.
However, Bolivia was too close to their former place. They were afraid the sea monsters would chase after them, so they had to go further to either Colombia or Venezuela. But that came with a huge risk as well because both of them were near the coast. Although they could stay a bit further from the coast, they might still be spotted by the monsters again sooner or later.
There was also another reason why they chose to go to this base. Unlike the base's situation, which still had a connection to the satellite, they didn't have anything to take advantage of. In other words, their civilization would be delayed while the US Base would advance by leaps and bounds.