Why does Ralph weep for Piggy and not for Simon at the end of "The Lord of the Flies?

I am not sure that the passage to which you are referring necessarily must be interpreted to mean that Ralph weeps more for Piggy than he does for Simon.  Although the last line of the paragraph -

"Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of the true, wise friend called Piggy" -

mentions Piggy by name and not Simon, it is the memory of Simon's death earlier in the same paragraph that causes Ralph to break down in the first place.  The author describes Ralph's state of mind at the end of the story by saying,

"the island was scorched up like dead wood - Simon was dead - and Jack had...the tears began to flow and sobs shook him.  He gave himself up to them now for the first time on the island; great shuddering spasms of grief that seemed to wrench his whole body".

I think it might well be argued that Ralph wept for both Simon and Piggy, both symbols of innocence and sacrifice whose lives were lost, victims of the baser nature of the human spirit (Chapter 12).