Because the traditional shape of the piece looks like a bishop's mitre.
Bishop...??? or is it Gajah?

Why is the rook named after a bird?
It looks like a castle or a tower, and the piece used to be a chariot.
Words evolve over time, and so do games.
Anyway, before the Bishop was the Bishop, it could only move 2 sqaures diagonally.
So they're different pieces.
EDIT: also, its only in English and Icelandic that Bishop means Bishop, in othe languages, it means runner or some such.
Or so Wikipedia tells me.

One theory I heard for the "elephant" having a diagonal movement pattern, which sounds plausible to me, is that the "elephant" is supposed to actually represent spear-throwers perched atop elephants. So the actual equivalent in terms of medieval battles would be an archer or longbowman or somesuch.

Why is the rook named after a bird?
According to Wikipedia, rook is borrowed from the Persian word rokh, meaning chariot.

The German Laufer means runner.
Bishops in medieval Europe were not royalty, but higher ranking than Knights in the social structure. It has nothing at all to do with religion, any more than the King promotes monarchy as a political system.
Get over it.
According to the definition I have... it's difficult to seperate bishop from religion...
It is also difficult to understand a Bishop's role in a fighting machine.
The predecessor to the current Bishop, "Gajah" was part of the 4 units (Chaturanga) which fought war in india.
Namely "elephants, chariots, cavalry, and infantry."*
The name change does not seem to have any relativity to the nature of the game.
(* according to Wikipedia)

The German Laufer means runner.
Bishops in medieval Europe were not royalty, but higher ranking than Knights in the social structure. It has nothing at all to do with religion, any more than the King promotes monarchy as a political system.
Get over it.
According to the definition I have... it's difficult to seperate bishop from religion...
It is also difficult to understand a Bishop's role in a fighting machine.
The predecessor to the current Bishop, "Gajah" was part of the 4 units (Chaturanga) which fought war in india.
Namely "elephants, chariots, cavalry, and infantry."*
The name change does not seem to have any relativity to the nature of the game.
(* according to Wikipedia)
Well, if you'd bothered to read my first reply, the name change is simply because of the shape of the piece. According to Wikipedia, the split in the top of the piece was supposed to represent stylized elephant tusks; over time, as it became increasingly stylized, it began to resemble a bishop's mitre. Incidentally, it was never called a "Gajah..." it was named "pil" or "al-Fil." There are only two languages in which it is called "bishop."

The German Laufer means runner.
It's the same in Swedish, the bishop here is known as löpare, which pretty much means runner.
And the knight is known as springare, which pretty much means runner.
=/

Listen Guys...
Sorry if I offended anyone... didnt mean it in to be that way.
I agree that Chess was not "religionized"(if I can say that). Bishop I guess was a character involved with war hence a natural successor.
Many regard the knight to be the most popular figure on the chess board and I still feel the Elephant would have been pretty on the board too
I wish to apologize to all those who responded politely and impersonally as the original post was not directed at any individual.
Throughout the history of chess, the game was never connected to religion.
Why then did the "Gajah" the War elephant in India become "Bishop" a religious character in europe.
May be this was an unfair evolution and should be changed back to Elephant or better still "Gajah".
After all its not fair to fix your own label on some one elses innovation.