Question

Why did bassanio decide not to choose the gold casket?

Answer 1

In  William Shakespeare's "The Merchant of Venice", Bassanio decided not to choose the gold casket as he felt that all that glitters is not gold. In other words, unlike the other suitors, he felt the outer appearance of the casket like that of a human being is not as important as the inner being and true love for a person,Portia in this case. So, he eventually chose the lead casket.  

Answer 2

Because Bassanio has different thinking than other suitors came there for choosing caskets. Bassanio has the thinking that we shouldn't look at the appearances because appearances are often deceptive. We should concentrate on inner beauty no matter how much beautiful and attractive is outside.
He left golden and silver caskets and chose lead casket ignoring its outer look and wins her as his wife.