In general, because they mature much more slowly. They happen to do a lot of their growing in the 6-18 month stage, especially in terms of filling out. They have a ton of size to make up, and if they are spayed / neutered too early it *can* (not always *will*) stunt their development a bit due to reduction of hormones.
That being said, the consensus seems to be that since giant breed females tend to go into heat later than your average dog (9+ months) you can wait a bit to get them fixed. Not everyone does this though, and there are a myriad of opinions out there.
I had one dane spayed at 10.5 months and the other spayed at 11 months, neither went into heat before we sent them in. With males it doesn't matter as much if you leave it later, but beware that after that 8 month - 1 yr mark they may start displaying unwanted male behaviours such as marking in the house.
Also FWIW, if you do own a giant breed I'd heavily consider getting a gastropexy done at the same time.