Importantly, please note that it did take hold in Hellenistic Bactria. This doesn't so much answer your question as refine it. Given that it was successful in Bactria, why not elsewhere?

There is pretty strong evidence that suggests Buddhist thought influenced Greek philosophy. In particular, the Pyrronhic school has a striking resemblance to certain types of Buddhist beliefs and philosophy.

It seems like the Greeks knew of Buddha

Can you please elaborate?

I believe looking at Ashoka's reign may shed some light. Ashoka ruled a huge chunk of the Indian subcontinent around 60 years after the battle between Alexander and Porus. He was a big proponent of Buddhism and was one of the first kings to spread Buddha's teachings. I think the emissaries that he sent to Sri Lanka and South East Asia were responsible for Buddhism gaining hold in those regions. But Ashoka also claimed to send many missionaries west ward - what happened to them I do not know.

There are a number of Ashoka's edicts all over the subcontinent and the westernmost one is at Kandahar - with the inscription in Greek and Aramaic.