[ History ] Open Question : Why did the Islamic invaders stop after conquering Afghanistan? why not further into India?

please explain what you can and please give dates in history as to when, where, and how much of Afghanistan Islamic tribes conquered? was this area already conquered before by Greeks? was this in essense a province of the "Roman Empire" as well? how many Romanians or Romans lived there and where? http://www.youtube.com/watch?v=gVtRiAQJtNo&feature=related are these people perhaps mixtures of Persians and Romans? are these called "Gypsies"? for how long lived in Afghanistan? and how influential today and why? please explain thanks for your answrs!