BobbyLee is correct. It was built by LG to Tracfones design specifications so as not to be useable with any other service. Modifying it would be expensive because it would be difficult if not impossible.
Unfortunately, Tracfones, Net10s and Gsm Straight Talk phones are generally a waste of time and effort to unlock. I would advise next time in the USA, you get a service that is much easier to unlock like Att gophone, Tmobile prepaid, Simple Mobile, Red Pocket, Air Voice, H20, Pure Prepaid, etc.