By the time Alexander defeated Porus, he was so far from Macedon that his troops rebelled. They were tired, exhausted and far from home, and they wanted to go back.
While Alexander's little crusade against the Persians appealed to the Greeks' historic enimity with the Persians, that didn't carry over to unknown kingdoms like Porus' one. The victory at Jhelum was also quite a costly one — but then, even that is skewed as some will tell you that it was a Greek victory and some will say that it was a Greek defeat.
Regardless of whether it was a victory or a defeat, the Greeks were tired of marching so far from familiar lands. And upon hearing that the plains beyond the Jhelum were home to an empire of innumerable infantry, cavalry and elephants, they had had enough and marched back.
So, to answer your question, exhaustion and low morale were probably the reasons why Alexander turned back at the banks of the India.
I hope this helps :)
(If this was the question to XP)
