Sounds like the ABS kicked in simply because the surface was very slippery or you had some grime on your tyres. Bear in mind that ABS is much easier to activate at slower speeds than it is at higher ones.
When ABS kicks in it often feels like you're not stopping as "hard" as you could be without it, as it feels like the car is preventing you from braking harder, if you see what I mean. In reality you're stopping as "hard" as you can for the conditions and, as previously mentioned, without ABS you'd have been skidding at that point and would have taken even longer to stop.
Whilst the main touted benefit of ABS is that it lets you steer whilst braking hard, a common misconception is that if you don't need to steer and are just stopping in a straight line, you'd stop the quickest by locking the wheels completely. This isn't the case, as a wheel which has maximum braking force applied yet is not skidding provides more stopping power than one which is locked.