There are two good reasons:
1:Pisces is the last sign of the Zodiac...which means it is a composite of everything that proceeded it. You can understand this comparison by looking at Aries, the first sign of the Zodiac, the first degree of 360, who typically has a lot of assertive energy, and...personality. Pisces receives the last degree of the Zodiac, 360. Inside of each Pisces there is a little bit of personality from all signs proceeding it...it's not that you don't have personality, it's just that you don't/don't have the ability, to disassociate yourself with your surroundings and from other people assertively enough.
2:Pisces is ruled by the Planet Neptune, which was the God of the Seas in Greek/Roman mythology. If you think of the characteristics of the sea, it is emotions, waves, deep unknown uncharted territories. It also makes up 70 percent of the World, so there's lots of elbow room for navigation...for a Pisces the world can be very big, and being the negative, water signs, they take all of that in unconsciously. Not only are they Water signs, they are Mutable water signs at that.
It's not that a Pisces doesn't have a personality; on the flip side, it's that they have so many they can't ever seem to just pick one.
