Because most decent grand pianos are made out of solid timber which is expensive and the lacquering of the wood is a long process too. Not to mention having to fine tune every single key by tightening or loosening the strings in the piano to the perfect note, and then making them stay that way.
They're is alot that goes into making a Grand. They have alot of parts and high quality grands are more expensive to manufacture. All of the material it takes is expensive too. It also takes alot to keep them up but they last a long time.
They are much larger and have a much more rich and professional sound. Although other pianos may sound nice, grand pianos have a very distictive sound and often don't need amplification even in the largest of theaters.