Water creates itself into a ball shape on iss?why?

It's actually the same reason that bubbles do.

The surface of the ball has surface tension, and that compresses the ball; the trick is; it turns out that the smaller the ball, the sharper the curvature, and so the higher the pressure inside it caused by surface tension.

So let's take an example, suppose you break the ball into two, with one bigger than the other.

Now touch them into each other, what's going to happen? The smaller ball is higher pressure, so it will decant itself, really quickly, into the bigger one, and the smaller one will shrink and the bigger one will grow until it's just one big ball.

Similar things happen with smaller distortions, basically, the ball hates sharp curvatures, because that implies higher pressure, so the sharp curvature areas rapidly get the water squeezed out of them, and they disappear until it's all the same curvature/pressure.

p.s. everything everyone else is saying in this thread is basically true as well, they're looking at it from a very high level view; what I've given is the worms-eye view of what's happening.

p.p.s. there's a table of pressures versus radius (of curvature) here:

https://en.wikipedia.org/wiki/Surface_tension#Surface_curvature_and_pressure