He knew that he was only here on a green card. He had plenty of time to engage in the process to become a citizen. He used all that time to make a good life for himself. One would think that he would have enough gratitude to become a citizen of the country that gave him the opportunity to be successful.
Well... If an immigrant, about the only things one gains from citizenship in the US are the right to vote, and freedom from deportation. Immigrants tend toward being Dems (although that is certainly not true of all!), so persuading them to remain green card holders is not all bad. Plus, some want to retire to their birth countries after their working years, and, that is not all bad either. Heck, finances alone have me wondering if I, a born in the US citizen, should head overseas once I’m no longer able to work effectively. Otherwise, I see no scenario where I do not have to work until I drop dead.