In the aftermath of Alexander the Great, the world was changing:
The vibrancy of the cultural exchange as Europe and Asia collided was astonishing. Statues of the Buddha started to appear only after the cult of Apollo became established in the Gundhara valley and western India. Buddhists felt threatened by the success of new religious practices and began to create their own visual images. Indeed, there is a correlation not only in the date of the earliest statues of the Buddha, but also in their appearance and design: it seems that it was Apollo that provided the template, such was the impact of Greek influences. Hitherto, Buddhists had actively refrained from visual representations; competition now forced them to react, to borrow and to innovate.
-Peter Frankopan, The Silk Roads: A New History Of The World