Why are Blasters always fitted with Caldari Navy Antimatter and not Void?

Antimatter is the T1 ammunition that does the highest damage, though with a -50% optimal range penalty. Void is T2 ammunition, so you need to be able to use T2 blasters to use it. It does more damage than CN antimatter under ideal conditions, but it also uses 25% more capacitor, has -25% optimal range and -50% Falloff, and a -25% tracking penalty.

The damage difference is also not that high, it is about 11-12%. While this is certainly significant, it is offset by the tracking penalty in particular which can easily make the applied dps much lower under real conditions.

Here is a damage graph for a Merlin shooting another frigate with Neutron Blasters while they are not moving at all:

Void does more damage up close, put even here you can already see that you pay for that with less flexibility in range. But this example is rather unrealistic, in most cases the frigates would be orbiting each other. And at orbit speeds of around 500 m/s for my example Merlin Void loses the advantage completely. Void also has a much lower distance range where it deals high damage, so you have to be more careful about setting your distance to the target:

But what happens when I shoot at something larger like a Battlecruiser and not another frigate? Here's the graph for that:

Void clearly is better in this case, the large signature radius of a Battlecruiser eliminates the tracking penalty.

In short, while Void is not a bad choice by any means, CN antimatter has fewer drawbacks, does similar damage or even more in many real situations and is generally more flexible. Void is still the best choice if you're shooting at a ship a class larger than your own, or if you know that tracking is likely not going to matter.

Personally I usually take some Void with me in addition to CN antimatter and Null (must have ammunition).