Are you considering Afghanistan East or West? Because it got there and in Central Asia too.

The spread of Buddhism from Afghanistan westward was countered, surprise surprise, by the spread of Islam.

Actually, it is quite interesting since it was brought in China trough the Silk Road by Iranic-speaking people. Many minorities escaped Persia when it was conquered by the Arabs, and many settled in less reachable places like Khotan and Turfan in what became the Chinese Turkestan (I don't know a thing about China, how it called that region now? Xinjang or something like it?). We have archaeological findings over there of Christian, Manichean and Buddhist texts being found over there, in the same areas, which make us think that they lived together before the cities died out.

Eventually the Turkic population moved in and de-Iranicized the region. They converted to Islam too rather quickly, so the border of Buddhism receded and prospered eastward.

As you can imagine, a religion whose focus is against pagan deities does not digest well a religion like Buddhism, but for a time it survived. The first Mongol rulers of Persia even brought with them some proselytizers. Eventually they became more and more connected with Islam (which makes sense if you have to rule over a Muslim populations) and Buddhism dwindled to non existence in the region. But something remained in the ancient monasteries in Afghanistan AND (as a small trivia) in the Persian word "bod", which means "idol". A remnant of past times probably born by the Muslim travelers (an iconoclast culture) going in Afghanistan and noticing the nice paintings of this "bod" (Buddhism by contrast as a very rich pictographic culture). In Persian poetry, being "beautiful as an idol" was something really nice that you would say to a loved one.