I suspect the people who abandon libertarianism never struggled to philosophically grasp first principles, and were instead drawn to the surface-level explanation of "socially liberal, fiscally conservative." With that kind of shallow grasp, it's no surprise the major parties can cajole them back. As for the Libertarian Party, that's been a lost cause for a long time, and it makes a mockery of libertarian principles just to win 3% of the total voter pool, which is utterly disgusting and probably repels many who seek a principled alternative to the major parties.