For too long the U.S. has taxed individual citizens on world-wide income from any source. This has caused such a hassle with ex-pats outside the country, that now many of them married to spouses of different nationalities have been faced with the choice of divorce or renouncing their citizenship.
It is too costly to maintain such a system with offices and difficult forms to fill out. Often, the spouse who is the citizen is not the one in charge of the finances, making the securing of the information to file stressful on the marriage.
This would encourage people to bring money back to the U.S. for investment purposes as our rates would then mirror the rest of the world's.