"He never made it as an actor."
Does it mean that he never became an actor or that he could never become successful as an actor ?
Is "make it" a fixed idiomatic expression or is there a rationale behind it ?
He never became successful.

'make it' is an idiom.
Yes, it's a fixed expression.
To make it as an X is also a fixed expression. The standard of success is a matter of opinion.
We might say, "He didn't quite make it," meaning his success was limited.
"He didn't make it as a plumber." Probably, he wasn't able to pay his bills.
In some endeavors, if you don't reach a certain plateau or threshold, you're a complete failure.