The main driver is talent base, ease of operations, and time zone overlaps.
The true value of offshoring comes from being able to essentially run 24/7 operations across a couple countries that have a strong talent base and relatively easy operations for us as a company (easy to fire, easy to hire, tax holidays, etc).
This is why Israel, India, Ireland, and parts of Eastern Europe like Romania, Poland, Belarus, and Ukraine became popular for software outsourcing.