A lot of people have been playing Philidor's Defense against me lately, and I was wonding why. It really allows white easy developement while it blocks in black's kingside bishop. Are people playing this to avoid the Ruy or other opeing systems, and if so why not play Petrov's Defense.

i often play that... i got many wins with this opening..at first black has a  crampled position but then it can make an attack..or a counterattack

Aftewhich, Black fianchettos his Bishop on g7, creating a King's Indian Defense... and then makes an attack on the Queen side.

I would think if somebody wanted a kings indian type defense the would play the pirc

For me, Pirc and Philidor-KID are cousins.

A matter of preference is regarding the King's Pawn. Some like Pirc maintaining the King's pawn at e7, or restraining it at e6, or later on pushing it to e5.

Some prefer Philidor, as they want to immediately block the e4 and e5 squares.  Of course, both intend to launch an attack on the Queen side, with the King's Bishop being maintained to protect the King's camp.