Why do you think George sarcastically says to Walter, "Good Night, Prometheus" in A Raisin in the Sun?

As George waits for Beneatha to change into "proper
attire" for the show, Walter talks to him about money and investments.  Walter is aware
of the financial success of the Murchison family, and he tells George that he must have
keen ideas about making money.  George wants little to do with this conversation, yet
Walter continues and tells him that they must get together to talk about business
plans.  George refuses the gesture, and Walter then rails him about his "useless"
education and ideas.  Walter claims that George knows nothing about being a man,
implying that he himself does.  On the way out, George calls Walter "Prometheus" as a
sarcastic stab at his identity and as an element of
foreshadowing. 

In Greek myth, Prometheus challenged the
power of the gods and thought that he was being clever by stealing fire for humankind. 
Similarly, Walter believes that his ideas trump those of his other family members
including Mama, and he simply sees the outside world as an oppressive force rather than
a path which he must learn to navigate.  For his actions, Prometheus is punished by the
gods and must live in the recurring hell of having his liver eaten by birds; Walter
later is punished by fate when he loses the family's money in an ill-planned scheme with
Willie Harris and Bobo.  Walter can never get this money back and is destined to live
with this guilt.

Walter, like Prometheus, is punished for
believing that he is clever enough to trump higher powers with his simple
plans.