According to the gravediggers in Hamlet, why is Ophelia to be given a Christian burial, even though her death may have been a suicide? Why would...

One gravedigger tells the other that Ophelia will get a Christian burial because she is a gentlewoman.

Ophelia apparently either slipped while picking flowers or deliberately jumped into the water.  It is generally assumed that she committed suicide because of the way in which she died.  Gertrude tells her brother that it was an accident, but that is just to appease Laertes, since they are worried about how emotional he is since his father’s death.  He is gunning for Hamlet, and the king and queen tried to calm him.

While Hamlet is hiding in the cemetery, he overhears the “clowns,” or gravediggers, discussing Ophelia’s death.

Second Clown

I tell thee she is: and therefore make her grave
straight: the crowner hath sat on her, and finds it
Christian burial.

First Clown

How can that be, unless she drowned herself in her
own defence? (Act 5, Scene 1)

This is comic relief, but it is also true.  Ophelia is granted a Christian burial in the cemetery, even though she should not get one if she committed suicide.  The gravediggers are making a joke about the inequality this represents.  Since Ophelia is a member of the royal court, she gets special privileges.

Second Clown

Will you ha' the truth on't? If this had not been
a gentlewoman, she should have been buried out o'
Christian burial. (Act 5, Scene 1)

Ophelia was definitely distraught leading up to her death, as a result of Hamlet’s callous treatment of her.  He told her that he never loved her and said some pretty awful things to her (like the part about going to a nunnery instead), as part of his play at madness.  He wants everyone to think he is crazy, but Ophelia is collateral damage.

The scene in the cemetery provides a chance for both comic relief and introspection.  Hamlet sees the skull of the former court jester, a man who used to make him laugh and played with him, and ponders again about the cruelty and brevity of life.  At the same time, we have the humorous interactions with the gravediggers to entertain us.  There is also the very real commentary about special privilege for the wealthy nobles.