Originally Posted by tetsu-katana
Correct me if I'm wrong, but weren't those areas actually Hindu, rather than Buddhist? Kabul is about 3000 years old from what I've read, and it was originally a Hindu settlement, but the Arabs destroyed it's Hindu culture in the 7th century when the Muslims went on their conquest. The mountains that run through Afghanistan are called Hindu Kush, which, I believe, is Persian for "Destruction of the Hindus".
The Greeks took over Bactria when Alexander took over the area. Bactria took on Greek culture and Diodotus I, in 256 BC, was made a satrap of the Seleucids. A little later he assumed complete independence. His successor, Euthydemus, successfully fought off attempts to bring Bactria back into the Seleucid empire and his son Demetrius made Bactria into a powerful state. Around the time that Buddhism was introduced in the 2nd century BC, the Seleucid ruler, Antiochus IV, sent Eucratidas into Bactria and Eucratidas brought about the death of Demetrius, though he was also killed soon after. One of Demetius's generals, Menander, ruled for a bit until he also died and the Kushans moved in around 130 BC, making Bactria part of the Kushan empire. Greek culture and Buddhism actually blended together well and thrived under Kushan rule, and there were few Hindus. It was when the White Huns, or Ephthalites, invaded Bactria in the 5th century, 200 years after the Kushan empire fragmented, when they utterly annihilated the Greco-Buddhist culture and left most of the country in ruins. That left it easy prey for the Sassanid Persians and then the Arabs.
There are two ways you can go about preserving Buddhism in the area: possibly have Demetrius survive Eucratidas and kill him first, allowing him to solidify his efforts at making Bactria strong and preserve a royal dynasty, allowing Greco-Buddhist culture to thrive under Bactrian rule. The Kushans wouldn't have found such a Bactria as easy to conquer and their empire wouldn't have lasted nearly as long without Bactria in it. Bactria, as a small centralized state, would likely have been able to fend off the Ephthalites too. Perhaps, fortified in the mountains of Afghanistan, Bactria would have been able to resist the Arabs too, not as farfetched as it might seem. Bactria really was in ruins both literally and culturally when the Muslims came along.
Or you can keep the Kushans from fragmenting. This could be harder, though. The Kushan empire grew quite large and unwieldy and I'm not too sure about the details under which the fragmentation took place. In fact, there are few detailed sources on Kushan history.